Isaac Newton Principia English version

MATH.- ST AT.

NEWTON’S PRINCIPLE

THE

MATHEMATICAL PRINCIPLES

OF

NATURAL PHILOSOPHY,

BY SIR ISAAC NEWTON;

11

TRANSLATED INTO ENGLISH BY ANDREW MOTTE.

TO WHICH IS ADDED

NEWTON’S SYSTEM OE THE WORLD;

With a Portrait taken from the Bust in the Royal Observatory at Greenwich.

FIRST AMERICAN EDITION, CAREFULLY REVISED AND CORRECTED,

WITH A LIFE OF THE AUTHOR, BY N. W. CHITTENDEN, M. A., &e.

NEW-YORK

PUBLISHED BY DANIEL ADEE, 45 LIBERTY STREET.

) i / ~/ v •

\

tatored according to Act of Congress, in the year 1846, by
DANIEL ADEE.

iLtht Clerk’s Office uf the Southern Disluct Court of New-York

Turney $ Lockwood ’« Sterwu.
16 Spruce St. N. Y.

DEDICATION.

TO THE

TEACHERS OF THE NORMAL SCHOOL

OF THE STATE OF NEW-YORK.

Gentlemen :—

A stirring freshness in the air, and ruddy streaks upon the
horizon of the moral world betoken the grateful dawning of a new
era. The days of a drivelling instruction are departing. With
us is the opening promise of a better time, wherein genuine man¬
hood doing its noblest work shall have adequate reward.
Teacher is the highest and most responsible office man can fill.
Its dignity is, and will yet be held commensurate with its duty—
a duty boundless as man’s intellectual capacity, and great as his
moral need—a duty from the performance of which shall emanate
an influence not limited to the now and the here> but which surely
will, as time flows into eternity and space into infinity, roll up, a
measureless curse or a measureless blessing, in inconceivable
swellings along the infinite curve. It is an office that should be
esteemed of even sacred import in this country. Ere long a hun¬
dred millions, extending from the Atlantic to the Pacific, from
Baffin’s Bay to that of Panama, shall call themselves American
citizens. What a field for those two master-passions of the hu¬
man soul—the love of Rule, and the love of Gain! How shall
our liberties continue to be preserved from the graspings of Am¬
bition and the corruptions of Gold ? Not by Bills of Rights

4

DEDICATION.

Constitutions, and Statute Books ; but alone by the rightly culti¬
vated hearts and heads of the People. They must themselves
guard the Ark. It is yours to fit them for the consecrated
charge. Look well to it: for you appear clothed in the majesty
of great power ! It is yours to fashion, and to inform , to save,
and to perpetuate. You are the Educators of the People : you
are the prime Conservators of the public weal. Betray your
trust, and the sacred fires would go out, and the altars crumble
into dust: knowledge become lost in tradition, and Christian no¬
bleness a fable! As you, therefore, are multiplied in number,
elevated in consideration, increased in means, and fulfill, well and
faithfully, all tfie requirements of true Teachers, so shall our fa¬
voured land lift up her head among the nations of the earth, and
call herself blessed.

In conclusion, Gentlemen, to you, as the conspicuous leaders
in the vast and honourable labour of Educational Reform, ana
Popular Teaching, the First American Edition of the Principia ol
Newton—the greatest work of the greatest Teacher—is most
respectfully dedicated.

N. W. CHITTENDEN.

INTRODUCTION TO THE AMERICAN EDITION.

That the Principia of Newton should have remained so gen¬
erally unknown in this country to the present day is a somewhat
remarkable fact; because the name of the author, learned with
the very elements of science, is revered at every hearth-stone
where knowledge and virtue are of chief esteem, while, abroad,
in all the high places of the land, the character which that name
recalls is held up as the noblest illustration of what Man may be,
and may do, in the possession and manifestation of pre-eminent
intellectual and moral worth ; because the work is celebrated, not
only in the history of one career and one mind, but in the history
of all achievement and human reason itself; because of the spirit
of inquiry, which has been aroused, and which, in pursuing its
searchings, is not always satisfied with stopping short of the foun¬
tain-head of any given truth ; and, finally, because of the earnest
endeavour that has been and is constantly going on, in many
sections of the Republic, to elevate the popular standard of edu¬
cation and give to scientific and other efforts a higher and a
better aim.

True, the Principia has been hitherto inaccessible to popular
use. A few copies in Latin, and occasionally one in English may
be found in some of our larger libraries, or in the possession of
some ardent disciple of the great Master. But a dead language
in the one case, and an enormous price in both, particularly in
that of the English edition, have thus far opposed very sufficient
obstacles to the wide circulation of the work. It is now, how¬
ever, placed within the reach of all. And in performing this la¬
bour, the utmost care has been taken, by collation, revision, and
otherwise, to render the First American Edition the most accurate
and beautiful in our language. “ Le plus beau monument que
f on puisse elever a la gloire de Newton, c’est une bonne edition
de ses ouvragesand a monument like unto that we would here

v;

INTRODUCTION TO

set up. The Principia, above all, glows with the immortality of
a transcendant mind. Marble and brass dissolve and pass away ;
but the true creations of genius endure, in time and beyond time,
forever : high upon the adamant of the indestructible, they send
forth afar and near, over the troublous waters of life, a pure, un¬
wavering. quenchless light whereby the myriad myriads of barques,
richly laden with reason, intelligence and various faculty, are
guided through the night and the storm, by the beetling shore
and the hidden rock, the breaker and the shoal, safely into havens
calm and secure.

To the teacher and the taught, the scholar and the student, the
devotee of Science and the worshipper of Truth, the Principia
must ever continue to be of inestimable value. If to educate
means, not so much to store the memory with symbols and facts,
as to bring forth the faculties of the soul and develope them to the
full by healthy nurture and a hardy discipline, then, what so effec¬
tive to the accomplishment of that end as the study of Geometri¬
cal Synthesis ? The Calculus, in some shape or other, is, indeed,
necessary to the successful prosecution of researches in the higher
branches of philosophy. But has not the Analytical encroached
upon the Synthetical, and Algorithmic Formulae been employed
when not requisite, either for the evolution of truth, or even its
apter illustration ? To each method belongs, undoubtedly, an
appropriate use. Newton, himself the inventor of Fluxions,
censured the handling of Geometrical subjects by Algebraical
calculations ; and the maturest opinions which he expressed were
additionally in favour of the Geometrical Method. His prefer¬
ence, so strongly marked, is not to be reckoned a mere matter of
taste ; and his authority should bear with preponderating weight
upon the decision of every instructor in adopting what may be
deemed the best plan to insure the completest mental develop-
hient. Geometry, the vigorous product of remote time ; blended
with the earliest aspirations of Science and the earliest applica¬
tions of Art; as well in the measures of music as in the move¬
ment of spheres; as wholly in the structure of the atom as in that
of the world; directing Motion and shaping Appearance; in a
word, the moulding of the created all, is, in comprehensive

THE AMERICAN EDITION.

VII

view, the outward form of that Inner Harmony of which and in
which all things are. Plainly, therefore, this noble study has
other and infinitely higher uses than to increase the power of ab¬
straction. A more general and thorough cultivation of it should
oe strenuously insisted on. Passing from the pages of Euclid or
Legendre, might not the student be led, at the suitable time, to
those of the Principia wherein Geometry may be found in varied
use from the familiar to the sublime ? The profoundest and the
happiest results, it is believed, would attend upon this enlargement
of our Educational System.

Let the Principia, then, be gladly welcomed into every Hall
where a true teacher presides. And they who are guided to
the diligent study of this incomparable work, who become
strengthened by its reason, assured by its evidence, and enlight¬
ened by its truths, and who rise into loving communion with the
great and pure spirit of its author, will go forth from the scenes
of their pupilage, and take their places in the world as strong-
minded, right-hearted men—such men as the Theory of our
Government contemplates and its practical operation absolutely
demands.

LIFE OF

SIR ISAAC NEWTON.

Nec fas est proprius mortali attingere Divos.— Halley.

From the thick darkness of the middle ages man’s struggling
spirit emerged as in new birth ; breaking out of the iron control
of that period ; growing strong and confident in the tug and din
of succeeding conflict * and revolution, it bounded forwards and
upwards with resistless vigour to the investigation of physical and
moral truth; ascending height after height; sweeping afar over
the earth, penetrating afar up into the heavens; increasing in en¬
deavour, enlarging in endowment; every where boldly, earnestly
out-stretching, till, in the Author of the Principia, one arose,
who, grasping the master-key of the universe and treading its
celestial paths, opened up to the human intellect the stupendous
realities of the material world, and, in the unrolling of its harmo¬
nies, gave to the human heart a new song to the goodness, wis¬
dom, and majesty of the all-creating, all-sustaining, all-perfect
God.

Sir Isaac Newton, in whom the rising intellect seemed to attain,
as it were, to its culminating point, was born on the 25th of De¬
cember, O. S. 1642—Christmas day—at Woolsthorpe, in the
parish of Colsterworth, in Lincolnshire. His father, John New¬
ton, died at the age of thirty-six, and only a few months after his
marriage to Harriet Ayscough, daughter of James Ayscough, ol
Rutlandshire. Mrs. Newton, probably wrought upon by the
early loss of her husband, gave premature birth to her only and
posthumous child, of which, too, from its extreme diminutiveness,
she appeared likely to be soon bereft. Happily, it was otherwise
decreed! The tiny infant, on whose little lips the breath of life

10

LIFE OF SIR ISAAC NEWTON.

so doubtingly hovered, lived ;—lived to a vigorous maturity, to a
hale old age ;—lived to become the boast of his country, the won¬
der of his time, and the “ornament of his species.”

Beyond the grandfather, Robert Newton, the descent of Sir
Isaac cannot with certainty be traced. Two traditions were held
in the family: one, that they were of Scotch extraction ; the
other, that they came originally from Newton, in Lancashire,
dwelling, for a time, however, at Westby, county of Lincoln, be¬
fore the removal to and purchase of Woolsthorpe—about a hundred
years before this memorable birth.

The widow Newton was left with the simple means of a com¬
fortable subsistence. The Woolsthorpe estate together with
small one which she possessed at Sewstern, in Leicestershire, yield
edher an income of some eighty pounds; ancl upon this limited sum,
she had to rely chiefly for the support of herself, and the educa
tion of her child. She continued his nurture for three years,
when, marrying again, she confided the tender charge to the care
of her own mother.

Great genius is seldom marked by precocious development;
and young Isaac, sent, at the usual age, to two day schools at
Skillington and Stoke, exhibited no unusual traits of character.
In his twelfth year, he was placed at the public school at Gran¬
tham, and boarded at the house of Mr. Clark, an apothecary.
But even in this excellent seminary, his mental acquisitions con¬
tinued for a while unpromising enough : study apparently had no
charms for him; he was very inattentive, and ranked low in the
school. One day, however, the boy immediately above our seem¬
ingly dull student gave him a severe kick in the stomach; Isaac,
deeply affected, but with no outburst of passion, betook himself,
with quiet, incessant toil, to his books ; he quickly passed above
the offending classmate ; yet there he stopped not; the strong
spirit was, for once and forever, awakened, and, yielding to its
noble impulse, he speedily took up his position at the head of all.

His peculiar character began now rapidly to unfold itself.
Close application grew to be habitual. Observation alternated
with reflection. “ A sober, silent, thinking lad,” yet, the wisest
and the kindliest, the indisputable leader of his fellows. Gener-

LIFE OF SIR ISA VC NEWTON.

11

osity, modesty, and a love of truth distinguished him then as ever
afterwards. He did not often join his classmates in play ; but he
would contrive for them various amusements of a scientific kind.
Paper kites he introduced; carefully determining their best form
and proportions, and the position and number of points whereby
to attach the string. He also invented paper lanterns ; these
served ordinarily to guide the way to school in winter mornings,
but occasionally for quite another purpose ; they were attached to
the tails of kites in a dark night, to the dismay of the country people
dreading portentous comets, and to the immeasureable delight ol
his companions. To him, however, young as he was, life seemed
to have become an earnest thing. When not occupied with his
studies, his mind would be engrossed with mechanical contrivances;
now imitating, now inventing. He became singularly skilful in the
use of his little saws, hatchets, hammers, and other tools. A
windmill was erected near Grantham ; during the operations ol
the workmen, he was frequently present; in a short time, he had
completed a perfect working model of it, which elicited general
admiration. Not content, however, with this exact imitation, he
conceived the idea of employing, in the place of sails, animal power ,
and, adapting the construction of his mill accordingly, he enclosed
in it a mouse, called the miller, and which by acting on a sort ot
treadwheel, gave motion to the machine. He invented, too, a
mechanical carriage—having four wheels, and put in motion with
a handle worked by the person sitting inside. The measurement
of time early drew his attention. He first constructed a water
clock, in proportions somewhat like an old-fashioned house clock.
The index of the dial plate was turned by a piece of wood acted
upon by dropping water. This instrument, though long used by
himself, and by Mr. Clark’s family, did not satisfy his inquiring
mind. His thoughts rose to the sun ; and, by careful and oft-re¬
peated observations of the solar movements, he subsequently
formed many dials. One of these, named Isaac's dial , was the
accurate result of years’ labour, and was frequently referred to
for the hour of'the day by the country people.

May we not discern in these continual efforts—the diligent re
search ; the patient meditation, the aspiring glance, and the energy

12

LIFE OF SIR ISAAC NEWTON.

of discovery—the stirring elements of that wondrous spirit,
which, clear, calm, and great, moved, in after years, through
deep onward through deep of Nature’s mysteries, unlocking her
strongholds, dispelling darkness, educing order—everywhere si¬
lently conquering.

Newton had an early and decided taste for drawing. Pictures,
taken sometimes from copies, but often from life, and drawn,
coloured and framed by himself, ornamented his apartment. He
was skilled also, in poetical composition, “ excelled in making
verses some of these were borne in remembrance and repeated,
seventy years afterward, by Mrs. Vincent, for whom, in early
youth, as Miss Storey, he formed an ardent attachment. She
was the sister of a physician resident near Woolsthorpe; but
Newton’s intimate acquaintance with her began at Grantham,
where they were both numbered among the inmates of the same
house. Two or three years younger than himself, of great per¬
sonal beauty, and unusual talent, her society afforded him the
greatest pleasure ; and their youthful friendship, it is believed,
gradually rose to a higher passion; but inadequacy of fortune
prevented their union. Miss Storey was afterwards twice mar¬
ried ; Newton, never; his esteem for her continued unabated
during life, accompanied by numerous acts of attention and
kindness.

In 1656, Newton’s mother was again left a widow r , and took
up her abode once more at Woolsthorpe. He was now fifteen
years of age, and had made great progress in his studies ; but she,
desirous of his help, and from motives of economy, recalled him
from school. Business occupations, however, and the manage¬
ment of the farm, proved utterly distasteful to him. When sent to
Grantham Market on Saturdays, he would betake himself to his
former lodgings in the apothecary’s garret, where ’some of Mr.
Clark’s old books employed his thoughts till the aged and trust¬
worthy servant had executed the family commissions and announced
the necessity of return : or, at other times, our young philosopher
would seat himself under a hedge, by the wayside, and continue
his studies till the same faithful personage—proceeding alone to
the town and completing the day’s business—stopped as he re-

LIFE OF SIR ISAAC NEWTON.

13

turned. The more immediate affairs of the farm received no
better attention. In fact, his passion for study grew daily more
absorbing, and his dislike for every other occupation more in¬
tense. His mother, therefore, wisely resolved to give him all the
advantages which an education could confer. He was sent back
to Grantham school, where he remained for some months in busy
preparation for his academical studies. At the recommendation
of one of his uncles, who had himself studied at Trinity College,
Cambridge, Newton proceeded thither, and was duly admitted,
on the 5th day of June 1660, in the eighteenth year of his age.

The eager student had now entered upon a new and wider
field; and we find him devoting himself to the pursuit of know¬
ledge with amazing ardour and perseverance. Among other sub¬
jects, his attention was soon drawn to that of Judicial Astrology
He exposed the folly of this pseudo-science by erecting a figure
with the aid of one or two of the problems of Euclid ;—and thus
began his study of the Mathematics. His researches into this
science were prosecuted with unparallelled vigour and success.
Regarding the propositions contained in Euclid as self-evident
truths, he passed rapidly over this ancient system—a step which
he afterward much regretted—and mastered, without further pre¬
paratory study, the Analytical Geometry of Descartes. Wallis’s
Arithmetic of Infinites, Saunderson’s Logic, and the Optics of
Kepler, he also studied with great care ; writing upon them
many comments; and, in these notes on Wallis’s work was un¬
doubtedly the germ of his fluxionary calculus. His progress was
so great that he found himself more profoundly versed than his tutor
in many branches of learning. Yet his acquisitions were not
gotten with the rapidity of intuition; but they were thoroughly
made and firmly secured. Quickness of apprehension, or intel
lectual nimbleness did not belong to him. He saw too far : his,
insight was too deep. He dwelt fully, cautiously upon the least
subject; while to the consideration of the greatest, he brought a
massive strength joined with a matchless clearness, that, regard¬
less of the merely trivial or unimportant, bore with unerring sa¬
gacity upon the prominences of the subject, and, grappling with
its difficulties, rarely failed to surmount them.

14

LIKE'OK SIR ISAAC NEWTON

His early and last friend, Dr. Barrow—in compass of inven¬
tion only inferior to Newton—who had been elected Professor
of Greek in the University, in 1660, was made Lucasian Profes¬
sor of Mathematics in 1663, and soon afterward delivered his
Optical Lectures : the manuscripts of these were revised by New¬
ton, and several oversights corrected, and many important sug¬
gestions made by him ; but they were not published till 1669.

In the year 1665, he received the degree of Bachelor of Arts;
and, in 1666, he entered upon those brilliant and imposing dis¬
coveries which have conferred inappreciable benefits upon science,
and immortality upon his own name.

Newton, himself, states that he was in possession of his Method
of Fluxions, “in the year 1666, or before.” Infinite quantities
had long been a subject of profound investigation; among the
ancients by Archimedes, and Pappus of Alexandria; among the
moderns by Kepler, Cavaleri, Roberval, Fermat and Wallis.
With consummate ability Dr. Wallis had improved upon the la¬
bours of his predecessors: with a higher power, Newton moved
forwards from where Wallis stopped. Our author first invented
his celebrated Binomial Theorem. And then, applying this
Theorem to the rectification of curves, and to the determination
of the surfaces and contents of solids, and the position of their
centres of gravity, he discovered the general principle of deducing
the areas of curves from the ordinate, by considering the area as
a nascent quantity, increasing by continual fluxion in the propor¬
tion of the length of the ordinate, and supposing the abscissa
to increase uniformly in proportion to the time. Regarding lines
as generated by the motion of points, surfaces by the motion of
lines, and solids by the motion of surfaces, and considering that
the ordinates, abscissae, &c., of curves thus formed, vary accord¬
ing to a regular law depending on the equation of the curve,
he deduced from this equation the velocities with which these
quantities are generated, and obtained by the rules of infinite
series, the ultimate value required. To the velocities with which
every line or quantity is generated, he gave the name of Flux¬
ions, and to the lines or quantities themselves, that of Fluents.
A discovery that successively baffled the acutest and strongest

15

LIFE OF SIR ISAAC NEWTON.

/

intellects :—that, variously modified, has proved of incalculable
service in aiding to develope the most abstruse and the highest
*ruths in Mathematics and Astronomy: and that was of itself
enough to render any name illustrious in the crowded Annals of
Science.

At this period, the most distinguished philosophers were direct¬
ing all their energies to the subject of light and the improvement
of the refracting telescope. Newton, having applied himself to
the grinding of “ optic glasses of other figures than spherical,” ex¬
perienced the impracticability of executing such lenses ; and con¬
jectured that their defects, and consequently those of refracting
telescopes, might arise from some other cause than the imperfect
convergency of rays to a single point. He accordingly “procured
a triangular glass prism to try therewith the celebrated phenom¬
ena of colours.” His experiments, entered upon with zeal, and
conducted with that industry, accuracy, and patient thought, lor
which he was so remarkable, resulted in the grand conclusion,
that Light was not homogeneous, but consisted of rays,

SOME OF WHICH WERE MORE REFRANGIBLE THAN OTHERS. This

profound and beautiful discovery opened up a new era in the
History of Optics. As bearing, however, directly upon the construc¬
tion of telescopes, he saw that a lens refracting exactly like a prism
would necessarily bring the different rays to different foci, at
different distances from the glass, confusing and rendering the
vision indistinct. Taking for granted that all bodies produced
spectra of * pial length, he dismissed all further consideration of
the refracting instrument, and took up the principle of reflection.
Rays of all colours, he found, were reflected regularly, so that the
angle of reflection was equal to the angle of incidence, and hence
he concluded that optical instruments might be brought to any
degree of perfection imaginable , provided reflecting specula of
the requisite figure and finish could be obtained. At this stage
of his optical researches, he was forced to leave Cambridge on
account of the plague which was then desolating England.

He retired to Woolsthorpe. The old manor-house, in which he
was born, was situated in a beautiful little valley, on the west side
of the river Witham ; and here in the quiet home of his boyhood,

2

16

LIFE OF SIR ISAAC NEWTON.

he passed his days in serene contemplation, while the stalking
pestilence was hurrying its tens of thousands into undistinguisha •
ble graves.

Towards the close of a pleasant day in the early autumn of
1666, he was seated alone beneath a tree, in his garden, absorbed
in meditation. He was a slight young man ; in the twenty-fourth
year of his age; his countenance mild and full of thought. For
a century previous, the science of Astronomy had advanced with
rapid strides. The human mind had risen from the gloom and
bondage of the middle ages, in unparalleled vigour, to unfold the
system, to investigate the phenomena, and to establish the laws
of the heavenly bodies. Copernicus, Tycho Brahe, Kepler,
Galileo, and others had prepared and lighted the way for him
who was ta give to their labour its just value, and to their genius
its true lustre. At his bidding isolated facts were to take order
as parts of one harmonious whole, and sagacious conjectures grow
luminous in the certain splendour of demonstrated truth. And
this ablest man had come—was here. His mind, familiar with
the knowledge of past effort, and its unequalled faculties develop¬
ed in transcendant strength, was now moving on to the very
threshold of Its grandest achievement. Step by step the untrod¬
den path was measured, till, at length, the entrance seemed dis¬
closed, and the tireless explorer to stand amid the first opening
wonders of the universe.

The nature of gravity—that mysterious power which causes
all bodies to descend towards the centre of the earth—had, in¬
deed, dawned upon him. And reason busily united link to link
of that chain which was yet to be traced joining the least to the
vastest, the most remote to the nearest, in one harmonious bond.
From the bottoms of the deepest caverns to the summits of the
highest mountains, this power suffers no sensible change : may not
its action, then, extend to the moon ? Undoubtedly : and furthej
reflection convinced him that such a power might be sufficient for
retaining that luminary in her orbit round the earth. But, though
this power suffers no sensible variation, in the little change of
distance from the earth’s centre, at which we may place our-
’blves, yet, at the distance of the moon, may not its force undergo

LIFE OF SIR ISAAC NEWTON.

17

more or less diminution ? The conjecture appeared most proba¬
ble : and, in order to estimate what the degree of diminution
might be, he considered that if the moon be retained in her orbit
by the force of gravity, the primary planets must also be carried
round the sun by the like power; and, by comparing the periods
of the several planets with their distances from the sun, he found
that, if they were held in their courses by any power like gravity,
its strength must decrease in the duplicate proportion of the in¬
crease of distance. In forming this conclusion, he supposed the
planets to move in perfect circles, concentric to the sun. Now
was this the law of the moon’s motion ? Was such a force, em¬
anating from the earth and directed to the moon, sufficient, when
diminished as the square of the distance, to retain her in her
orbit ? To ascertain this master-fact, he compared the space
through which heavy bodies fall, in a second of time, at a given
distance from the centre of the earth, namely, at its surface, with
the space through which the moon falls, as it were, to the earth,
in the same time, while revolving in a circular orbit. He was^
absent from books ; and, therefore, adopted, in computing the
earth’s diameter, the common estimate of sixty miles to a degree
of latitude as then in use among geographers and navigators.
The result of his calculations did not, ot course, answer his ex¬
pectations ; hence, he concluded that some other cause, beyond the
reach of observation—analogous, perhaps, to the vortices of Des¬
cartes—-joined its action to that of the power of gravity upon the
moori. Though by no means satisfied, he yet abandoned awhile
further inquiry, and remained totally silent upon the subject.

These rapid marches in the career of discovery, combined with
the youth of Newton, seem to evince a penetration the most
lively, and an invention the most exuberant. But in him there
was a conjunction of influences as extraordinary as fortunate.
Study, unbroken, persevering and profound carried on its inform¬
ing and disciplining work upon a genius, natively the greatest,
and rendered freest in its movements, and clearest in its vision,
through the untrammelling and enligl tening power of religion.
And, in this happy concurrence, are to be sought the elements of
those amazing abilities, which, grasping, with equal facility, the

18

LIFE OF SIR ISAAC NEWTON.

minute and the stupendous, brought these successively to light,
and caused science to make them her own.

In 1667, Newton was made a Junior Fellow; and, in the year
following, he took his degree of Master of Arts, and was appoint¬
ed to a Senior Fellowship.

On his return to Cambridge, in 1668, he resumed his optical
labours. Having thought of a delicate method of polishing metal,
he proceeded to the construction of his newly projected reflect'
ing telescope ; a small specimen of which he actually made with
his own hands. It was six inches long ; and magnified about
forty times ;—a power greater than a refracting instrument of six
feet tube could exert with distinctness. Jupiter, with his four
satellites, and the horns, or moon-like phases of Venus were
plainly visible through it. This was the first reflecting

TELESCOPE EVER EXECUTED AND DIRECTED TO THE HEAVENS.

He gave an account of it, in a letter to a friend, dated February 23d,
1668-9—a letter which is also remarkable for containing the firs'
allusion to his discoveries “ concerning the nature of light.” En¬
couraged by the success of his first experiment, he again executed
with his own hands, not long afterward, a second and superior
instrument of the same kind. The existence of this having come
to the knowledge of the Royal Society of London, in 1671, they
requested it of New r ton for examination. He accordingly sent it
to them. It excited great admiration ; it was shown to the king*
a drawing and description of it was sent to Paris; and the tele¬
scope itself was carefully preserved in the Library of the Society.
Newton lived to see his invention in public use, and of eminent
service in the cause of science.

In the spring of 1669, he wrote to his friend Francis Aston,
Esq., then about setting out on his travels, a letter of advice and
directions, it was dated May 18th, and is interesting as exhibit¬
ing some of the prominent features in Newton’s character.
Thus:—

“ Since in your letter you give me so much liberty of spending
my judgment about what may be to your advantage in travelling,
1 shall do it more freely than perhaps otherwise would have been
decent. First, then, I will lay down some general rules, most of

LIFE OF SIR ISAAC NEWTON.

19

which, I belie* e, you have considered already; but if any of
them be new to you, they may excuse the rest ; if none at all,
yet is my punishment more in writing than yours in reading.

“When you come into any fresh company. 1. Observe their
humours. 2. Suit your own carriage thereto, by which insinua¬
tion you will make their converse more free and open. 3. Let
your discourse be more in queries and doubtings than peremptory
assertions or disputings, it being the design of travellers to learn,
not to teach. Besides, it will persuade your acquaintance that
you have the greater esteem of them, and so make them more
ready to communicate what they know to you ; whereas nothing
sooner occasions disrespect and quarrels than peremptoriness.
You will find little or no advantage in seeming wiser or much
more ignorant than your company. 4. Seldom discommend any¬
thing though never so bad, or do it but moderately, lest you be
unexpectedly forced to an unhandsome retraction. It is safef to
commend any thing more than it deserves, than to discommend
a thing so much as it deserves; for commendations meet not
so often with oppositions, or, at least, are not usually so ill re¬
sented by men that think otherwise, as discommendations; and
you will insinuate into men’s favour by nothing sooner than seem¬
ing to approve and commend what they like; but beware o
doing it by comparison. 5. If you be affronted, it is better, in £
foreign country, to pass it by in silence, and with a jest, though
with some dishonour, than to endeavour revenge; for, in the first
case, your credit’s ne’er the worse when you return into England,
or come into other company that have not heard of the quarrel.
But, in the second case, you may bear the marks of the quarrel
while you live, if you outlive it at all. But, if you find yoursell
unavoidably engaged, ’tis best, I think, if you can command your
passion and language, to keep them pretty evenly at some certain
moderate pitch, not much heightening them to exasperate your
adversary, or provoke his friends, nor letting them grow overmuch
dejected to make him insult. In a word, if you can keep reason
above passion, that and watchfulness will be your best defendants.
To which purpose you may consider, that, though such excuses
as this—He provok’t me so much I could not forbear—may pass

20

LIFE OF SIR ISAAC NEWTON.

among friends, yet amongst strangers they are insignificant, ana
only argue a traveller’s weakness.

“ To these I may add some general heads for inquiries or ob¬
servations, such as at present I can think on. As, 1. To observe
the policies, wealth, and state affairs of nations, so far as a soli¬
dary traveller may conveniently do. 2. Their impositions upon
all sorts of people, trades, or commodities, that are remarkable.
3. Their laws and customs, how far they differ from ours. 4.
Their trades and arts wherein they excel or come short of us in
England. 5. Such fortifications as you shall meet with, their
fashion, strength, and advantages for defence, and other such mili¬
tary affairs as are considerable. 6. The power and respect be¬
longing to their degrees of nobility or magistracy. 7. It will not
be time misspent to make a catalogue of the names and excellen¬
cies of those men that are most wise, learned, or esteemed in any
nation. 8. Observe the mechanism and manner of guiding ships.

9. Observe the products of Nature in several places, especially in
mines, with the circumstances of mining and of extracting metals
or minerals out of their ore, and of refining them ; and if you
meet with any transmutations out of their own species into
another (as out of iron into copper, out of any metal into quick¬
silver, out of one salt into another, or into an insipid body, &c.),
those, above all, will be worth your noting, being the most lucif-
erous, and many times lucriferous experiments, too, in philosophy.

10. The prices of diet and other things. 11. And the staple
commodities of places.

“ These generals (such as at present I could think of), if they
will serve for nothing else, yet they may assist you in drawing up
a model to regulate your travels by. ’ As for particulars, these that
follow are all that I can now think of, viz.; whether at Schem-
nitium, in Hungary (where there are mines of gold, copper, iron,
vitriol, antimony, &e.). they change iron into copper by dissolving
A in-a vitriolate water, which they find in cavities of rocks in the
mines, and then melting the slimy solution in a stroi ig fire, which
in the cooling proves copper. The like is said to be done in other
places, which I cannot now remember; perhaps, too, it may be
lone in Italy. For about twenty or thirty years agone there was

LIFE OF SIR ISAAC NEWTON.

21

a certain vitriol came from thence (called Roman vitriol), but of
a nobler virtue than that which is now called by that name ;
which vitriol is not now to be gotten, because, perhaps, they make
a greater gain by some such trick as turning iron into copper
with it than by selling it. 2. Whether, in Hungary, Sclavonia,
Bohemia,, near the town Eila, or at the mountains of Bohemia
near Silesia, there be rivers whose waters are impregnated with
gold ; perhaps, the gold being dissolved by some corrosive water
like aqua regis , and the solution carried along with the stream,
that runs through the mines. And whether the practice of laying
mercury in the rivers, till it be tinged with gold, and then strain¬
ing the mercury through leather, that the gold may stay behind,
be a secret yet, or openly practised. 3. There is newly con¬
trived, in Holland, a mill to grind glasses plane withal, and I
think polishing them too ; perhaps it will be worth the while to see

it. 4. There is in Holland one-Borry, who some years since

was imprisoned by the Pope, to have extorted from him secrets
(as I am told) of great worth, both as to medicine and profit, but
he escaped into Holland, where they have granted him a guard.
I think he usually goes clothed in green. Pray inquire what you
can of him, and whether his ingenuity be any profit to the Dutch.
You may inform yourself whether the Dutch have any tricks to
keep their ships from being all worm-eaten in their voyages to
the Indies. Whether pendulum clocks do any service in finding
out the longitude, &c.

“ I am very weary, and shall not stay to part with a long
compliment, only I wish you a good journey, and God be with
you.”

It was not till the month of June, 1669, that our author made
known his Method of Fluxions. He then communicated the
work which he had composed upon the subject, and entitled,
Analysis per Equationes nu3iero terminorum Infinitas,
to his friend Dr. Barrow. The latter, in a letter dated 20th of the
same month, mentioned it to Mr. Collins, and transmitted it to
him, on the 31st of July thereafter. Mr. Collins greatly approv¬
ed of the work; took a copy of it; and sent the original back
to Dr. Barrow. During the same and the two following years, Mr

22

LIFE OF SIR ISAAC NEWTON.

Collins, by his extensive correspondence, spread the knowledge
of this discovery among the mathematicians in England, Scotland,
France, Holland and Italy.

Dr. Barrow, having resolved to devote himself to Theology,
resigned the Lucasian Professorship of Mathematics, in 1669, in
favour of Newton, who accordingly received the appointment to
the vacant chair.

During the years 1669, 1670, and 1671, our author, as such
Professor, delivered a course of Optical Lectures. Though these
contained his principal discoveries relative to the different re-
frangibility of light, yet the discoveries themselves did not be¬
come publicly known, it seems, till he communicated them to the
Royal Society, a few weeks after being elected a member there¬
of, in the spring of 1671-2. He now rose rapidly in reputation,
and was soon regarded as foremost among the philosophers of the
age. His paper on light excited the deepest interest in the Royal
Society, who manifested an anxious solicitude to secure the author
from the “ arrogations of others,” and proposed to publish his
discourse in the monthly numbers in which the Transactions were
given to the world. Newton, gratefully sensible of these expres¬
sions of esteem, willingly accepted of the proposal for publication.
He gave them also, at this time, the results of some further ex¬
periments in the decomposition and re-composition of light:—that
the same degree of refrangibility always belonged to the same
colour, and the same colour to the same degree of refrangibility:
that the seven different colours of the spectrum were original, or
simple, and that whiteness, or white light was a compound of all
these seven colours.

The publication of his new doctrines on light soon called forth
violent opposition as to their soundness. Hooke and Huygens—
men eminent for ability and learning—were the most conspicuous
of the assailants. And though Newton effectually silenced all his
adversaries, yet he felt the triumph of little gain in comparison
with the loss his tranquillity had sustained. He subsequently re-
narked in allusion to this controversy—and to one with whom
he was destined to have a longer and a bitterer conflict—“ I was
so persecuted with discussions arising from the publication 6f my

LIFE OF SIR ISAAC NEWTON.

23

theory ol light, that I blamed my own imprudence for parting
with so substantial a blessing as my quiet to run after a shadow/'

In a communication to Mr. Oldenburg, Secretary of the Royal
Society, in 1672, our author stated many valuable suggestions re¬
lative to the construction of Reflecting Microscopes which he
considered even more capable of improvement than telescopes.
He also contemplated, about the same time, an edition of Kinck-
huysen’s Algebra, with notes and additions; partially arranging,
as an introduction to the work, a treatise, entitled, A Method of
Fluxions; but he finally abandoned the design. This treatise,
however, he resolved, or rather consented, at a late period of his
life, to put forth separately ; and the plan would probably have
been carried into execution had not his death intervened. It was
translated into English, and published in 1736 by John Colson,
Professor of Mathematics in Cambridge.

Newton, it is thought, made his discoveries concerning the
Inflection and Diffraction of light before 1674. The phe¬
nomena of the inflection of light had been first discovered more
than ten years before by Grimaldi. And Newton began by re¬
peating one of the experiments of the learned Jesuit—admitting
a beam of the sun’s light through a small pin hole into a dark
chamber: the light diverged from the aperture in the form of a
cone, and the shadows of all bodies placed in this light were
larger than might have been expected, and surrounded with three
coloured fringes, the nearest being widest, and the most remote
the narrowest. Newton, advancing upon this experiment, took
exact measures of the diameter of the shadow r of a human hair,
and of the breadth of the fringes, at different distances behind it,
and discovered that these diameters and breadths were not pro¬
portional to the distances at which they were measured. He
hence supposed that the.rays which passed by the edge of the
hair were deflected or turned aside from it, as if by a repulsive
force, the nearest rays suffering the greatest, the more remote a
less degree of deflection. In explanation of the coloured fringes,
he queried : whether the rays which differ in refrangibility do not
differ also in flexibility, and whether they are m»t, by these dif¬
ferent inflections, separated from one another, so as after separa-

LIFE OF SIR ISAAC NEWTON.

^4

tion to make the colours in the three fringes above described i
Also, whether the rays, in passing by the edges and sides oi
bodies, are not bent several times backwards and forwards with
an eel-like motion—the three fringes arising from three such
bendings ? His inquiries on this subject were here interrupted
and never renewed.

His Theory of the Colours of Natural Bodies was commu*
nicated to the Royal Society, in February, 1675. This is justly
regarded as one of the profoundest of his speculations. The fun¬
damental principles of the Theory in brief, are:—That bodies
possessing the greatest refractive powers reHect the greatest
quantity of light; and that, at the confines of equally refracting
media, there is no reflection. That the minutest particles of al¬
most all natural bodies are in some degree transparent. That
between the particles of bodies there are pores, or spaces, either
empty or filled with media of a less density than the particles
themselves. That these particles, and pores or spaces, have some
definite size. Hence he deduced the Transparency, Opacity, and
colours of natural bodies. Transparency arises from the particles
and their pores being too small to cause reflection at their com¬
mon surfaces—the light all passing through ; Opacity from the
opposite cause of the particles and their pores being sufficiently
large to reflect the light which is “ stopped or stifled 7 ’ by the
multitude of reflections; and colours from the particles, accord¬
ing to their several sizes, reflecting rays of one colour and trans¬
mitting those of another—or in other words, the colour that
meets the eye is the colour reflected, while all the other rays are
transmitted or absorbed.

Analogous in origin to the colours of natural bodies, he con¬
sidered the colours of thin plates. This subject was interest¬
ing and important, and had attracted considerable investigation.
He, however, was the first to determine the law of the produc¬
tion of these colours, and, during the same year made known the
results of his researches herein to the Royal Society. His mode
of procedure in these experiments was simple and curious. He
placed a double convex lens of a large known radius of curvature,
rptfn the flat surface of a plano-convex object glass. Thus, from

LIFE OF SIR ISAAC NEWTON.

25

their point of contact at the centre, to the circumference of the
lens, he obtained plates of air, or spaces varying from the ex-
tremest possible thinness, by slow degrees, to a considerable thick¬
ness. Letting the light fall, every different thickness of this
plate of air gave different colours—the point of contact of the
lens and glass forming the centre of numerous concentric colored
liags. Now the radius of curvature of the lens being known, the
thickness of the plate of air, at any given point, or where any par¬
ticular colour appeared, could be exactly determined. Carefully
noting, therefore, the order in which the different colours ap¬
peared, he measured, with the nicest accuracy, the different thick*
nesses at which the most luminous parts of the rings were pro¬
duced, whether the medium were air, water, or mica—all these
substances giving the same colours at different thicknesses;—the
ratio of which he also ascertained. From the phenomena obser¬
ved in these experiments, Newton deduced his Theory of Fits of
Easy Reflection and Transmission of light. It consists in suppos¬
ing that every particle of light, from its first discharge from a lumi¬
nous body, possesses, at equally distant intervals, dispositions to
be reflected from, or transmitted through the surfaces of bodies
upon which it may fall. For instance, if the rays are in a Fit of
Easy Reflection, they are on reaching the surface, repelled,
thrown off, or reflected from it; if, in a Fit of Easy Transmission,
they are attracted, drawn in, or transmitted through it. By this
Theory of Fits, our author likewise explained the colours of
thick plates.

He regarded light as consisting of small material particles
emitted from shining substances. He thought that these parti¬
cles could be re-combined into solid matter, so that “ gross bodies
and light were convertible into one anotherthat the particles of
light and the particles of solid bodies acted mutually upon each
other; those of light agitating and heating those of solid bodies,
and the latter attracting and repelling the former. Newton was
the first to suggest the idea of the Polarization of light.

In the paper entitled An Hypothesis Explaining Properties oj
Light, December, 1675, our author first introduced his opinions re¬
specting Ether—opinions which he afterward abandoned and again

26

LIFE OF SIR S A.AC JSEWTON.

permanently resumed—“ A most subtle spirit which pervades” ah
bodies, and is expanded through all the heavens. It is electric,
and almost, if not quite immeasurably elastic and rare. “ By the
force and action of which spirit the particles of bodies mutually
attract one another, at near distances, and cohere, if contiguous ;
and electric bodies operate at greater distances, as well repelling
33 attracting the neighbouring corpuscles ; and light is emitted,
-reflected, refracted, inflected and heats bodies; and all sensation
is excited, and the members of animal bodies move at the com¬
mand of the will, namely, by the vibrations of this spirit, mutu¬
ally propagated along the solid filaments of the nerves, from the
outward organs of sense to the brain, and from the brain into the
muscles.” This “ spirit” was no anima mundi ; nothing further
from the thought of Newton ; but was it not, on his part, a par¬
tial recognition of, or attempt to reach an ultimate material force,
or primary element, by means of which, 1 in the roaring loom of
time,” this material universe, God’s visible garment, may be
woven for us ?

The Royal Society were greatly interested in the results of
some experiments, which our author had, at the same time, com¬
municated to them relative to the excitation of electricity in glass ;
and they, after several attempts and further direction from him,
succeeded in re-producing the same phenomena.

One of the most curious of Newton’s minor inquiries related to
the connexion between the refractive powers and chemical com¬
position of bodies. He found on comparing the refractive powers
and the densities of many different substances, that the former
were very nearly proportional to the latter, in the same bodies.
Unctuous and sulphureous bodies were noticed as remarkable excep¬
tions—as well as the diamond —their refractive powers being two
or three times greater in respect of their densities than in the
case of other substances, while, as among themselves, the one was
generally proportional to the other. He hence inferred as to the
diamond a great degree of combustibility ;—a conjecture which
the experiments of modern chemistry have shown to be true.

The chemical researches of our author were probably pursued
with more or less diligence from the time of his witnessing some

LIFE OF SIR ISAAC NEWTON.

27

vt' the uractical operations in that science at the Apothecary’s at
Grantham. De Natura Acidorum is a short chemical paper, on
various topics, and published in Dr. Horsley’s Edition of his
works. Tabula Quantitatum et Graduum Coloris was in¬
serted in the Philosophical Transactions ; it contains a compara¬
tive scale of temperature from that of melting ice to that of a
small kitchen coal-fire. He regarded fire as a body heated so hot
as to emit light copiously; and flame as a vapour, fume, or ex¬
halation heated so hot as to shine. To elective attraction, by
the operation of which the small particles of bodies, as he con¬
ceived, act upon one another, at distances so minute as to escape
observation, he ascribed all the various chemical phenomena ot
precipitation, combination, solution, and crystallization, and the
mechanical phenomena of cohesion and capillary attraction. New
ton’s chemical views were illustrated and confirmed, in part, at
least, in his own life-time. As to the structure of bodies, he was
of opinion “ that the smallest particles of matter may cohere by
the strongest attractions, and compose bigger particles of weaker
virtue ; and many of these may cohere and compose bigger par
tides whose virtue is still weaker; and so on for divers succes¬
sions, until the progression end in the biggest particles, on which
the operations in chemistry and the colours of natural bodies de¬
pend, and which by adhering, compose bodies of sensible magni¬
tude.”

There is good reason to suppose that our author was a diligent
student of the writings of Jacob Behmen ; and that in conjunction
with a relative, Dr. Newton, he was busily engaged, for several
months in the earlier part of life, in quest of the philosopher’s
tincture. “ Great Alchymist,” however, very imperfectly de¬
scribes the character of Behmen, whose researches into things
material and things spiritual, things human and things divine, ai-
ford the strongest evidence of a great and original mind.

More appropriately here, perhaps, than elsewhere, may be
given Newton’s account of some curious experiments, made in his
own person, on the action of light upon the retina. Locke, who
was an intimate friend of our author, wrote to him for his opinion
on a certain fact stated in Boyle’s Book of Colours. Newton, in

28

LIFE OF SIB ISAAC NEWTON.

his reply, dated June 30th, 1601, narrates the following circum¬
stances, which probably took place in the course of his optical
researches. Thus:—

“ The observation you mention in Mr. Boyle’s Book of Colours
I once tried upon myself with the hazard of my eyes. The
manner was this; I looked a very little while upon the sun in the
looking-glass with my right eye, and then turned my eyes into a
dark corner of my chamber, and winked, to observe the impres¬
sion made, and the circles of colours which encompassed it, and
how they decayed by degrees, and at last vanished. This I re¬
peated a second and a third time. At the third time, when the
phantasm of light and colours about it were almost vanished, in¬
tending my fancy upon them to see their last appearance, I found,
to my amazement, that they began to return, and by little and
little to become as lively and vivid as when I had newly looked
upon the sun. But when I ceased to intend my fancy upon them,
they vanished again. After this, I found, that as often as I went
into the dark, and intended my mind upon them, as when a man
looks earnestly to see anything which is difficult to be seen, I
could make the phantasm return without looking any more upon
the sun; and the oftener I made it return, the more easily I could
make it return again. And, at length, by repeating this, without
looking any more upon the sun, I made such an impression on my
eye, that, if I looked upon the clouds, or a book, or any bright
object, I saw upon it a round bright spot of light like the sun,
and, which is still stranger, though I looked upon the sun with
my right eye only, and not with my left, yet my fancy began f o
make an impression upon my left eye, as well us upon my right.
For if I shut my right eye, or looked upon a book, or the clouds,
with my left eye, I could see the spectrum of the sun almost as
plain as with my right eye, if I did but intend my fancy a little
while upon it; for at first, if I shut my right eye, and looked with
my left, the spectrum of the sun did not appear till I intended my
fancy upon it; but by repeating, this appeared every time more
easily. And now, in a few hours’ time, I had brought my eyes
to such a pass, that I could look upon no blight object with either
eye, but I saw the sun before me, so that I durst neither write

LIFE OF SIR ISAAC NEWTON.

29

nor read ; but to recover the use of my eyes, shut myself up in
my chamber made dark, for three days together, and used all
means to divert my imagination from the sun. For if I thought
upon him, I presently saw his picture, though I was in the dark.
But by keeping in the dark, and employing my mind about other
things, I began in three or four days to have some use of my eyes
again; and by forbearing to look upon bright objects, recovered
them pretty well, though not so well but that, for some months
after, the spectrum of the sun began to return as often as I began
to meditate upon the phenomena, even though I lay in bed at mid¬
night with my curtains drawn. But now I have been very well
for many years, though I am apt to think, if I durst venture my
eyes, I could still make the phantasm return by the power of my
fancy. This story I tell you, to let you understand, tha]; in the
observation related by Mr. Boyle, the man’s fancy probably con¬
curred with the impression made by the sun’s light to produce
that phantasm of the sun which he constantly saw in bright ob¬
jects. And so your question about the cause of phantasm in¬
volves another about the power of fancy, which I must confess is
too hard a knot for me to untie. To place this effect in a constant
motion is hard, because the sun ought then to appear perpetually.
It seems rather to consist in a disposition of the sensorium to
move the imagination strongly, and to be easily moved, both by
the imagination and by the light, as often as bright objects are
looked upon/’

Though Newton had continued silent, yet his thoughts were
by no means inactive upon the vast subject of the planetary mo¬
tions. The idea of Universal Gravitation, first caught sight of, so
to speak, in the garden at Woolsthorpe, years ago, had gradually
expanded upon him. We find him, in a letter to Dr. Hooke,
Secretary of the Royal Society, dated in November, 1679, pro¬
posing to verify the motion of the earth by direct experimem,
namely, by the observation of the path pursued by a body falling
from a considerable height. He had concluded that the path
would be spiral; but Dr. Hooke maintained that it would be an
eccentric •ellipse in vacuo, and an ellipti-spiral in a resisting me¬
dium. Our author, aided by this correction of his error, and by

30

LIFE OF SIR ISAAC NEWTON.

the discovery that a projectile would move in an elliptical orbil
when under the influence of a force varying inversely as the
square of the distance, was led to discover “ the theorem by
which he afterwards examined the ellipsis f and to demonstrate
the celebrated proposition that a planet acted upon by an attrac¬
tive force varying inversely as the squares of the distances will
describe an elliptical orbit, in one of whose foci the attractive
force resides.

When he was attending a meeting of the Royal Society, in
June 1682, the conversation fell upon the subject of the measure¬
ment of a degree of the meridian, executed by M. Picard, a
French Astronomer, in 1679. Newton took a memorandum oi
the result; and afterward, at the earliest opportunity, computed
from it the diameter of the earth : furnished with these new data,
he resumed his calculation of 1666. As he proceeded therein,
he saw that his early expectations were now likely to be realized :
the thick rushing, stupendous results overpowered him; he be¬
came unable to carry on the process of calculation,, and intrusted
its completion to one of his friends. The discoverer had, indeed,
grasped the master-fact. The law of falling bodies at the earth's
surface w T as at length identified with that which guided the moon
in her orbit. And so his Great Thought, that had for sixteen
years loomed up in dim, gigantic outline, amid the first dawn of a
plausible hypothesis, now stood forth, radiant and not less grand,
in the mid-day light of demonstrated truth.

It were difficult, nay impossible to imagine, even, the influence
of a result like this upon a mind like Newton’s. It was as if the
keystone had been fitted to the glorious arch by which his spirit
should ascend to the outskirts of infinite space—spanning the immea¬
surable—weighing the imponderable—computing the incalculable
—mapping out the marchings of the planets, and the far-wander-
ings of the corners, and catching, bring back to earth some clearer
notes of that higher melody which, as a sounding voice, bears
perpetual witness to the design and omnipotence of a creating
Deity.

Newton, extending the law thus obtained, composed a series
of about twelve propositions on the motion of the primary planets

LIFE OF SIR ISAAC NEWTON.

31

about the sun. These were sent to London, and communicated
to the Royal Society about the end of 1683. At or near this pe¬
riod, other philosophers, as Sir Christopher Wren, Dr. Halley,
and Dr. Hooke, were engaged in investigating the same subject;
but with no definite or satisfactory results. Dr. Halley, having
seen, it is presumed, our author’s propositions, went in August,
1684, to Cambridge to consult with him upon the subject.
Newton assured him that he had brought the demonstration to
perfection. In November, Dr. Halley received a copy of the
work; and, in the following month^ announced .it to the Royal
Society, with the author’s promise to have it entered upon their
Register. Newton, subsequently reminded by the Society of his
promise, proceeded in the diligent preparation of the work, and.
though suffering an interruption of six weeks, transmitted the
manuscript of the first book to London before the end of April.
The work was entitled Philosophise Naturalis Principia
Mathematica, dedicated to the Royal Society, and presented
thereto on the 28th of April, 1685-6. The highest encomiums
were passed upon it; and the council resolved, on the 19th of
May, to print it at the expense of the Society, and under the di¬
rection of Dr. Halley. The latter, a few days afterward, com¬
municated these steps to Newton, who, in a reply, dated the 20th
of June, holds the following language :—“ The proof you sent me
I like very well. I designed the whole to consist of three books ;
the second was finished last summer, being short, and only wants
transcribing, and drawing the cuts fairly. Some new propositions
I have since thought on, which I can as well let alone. The
third wants the theory of comets. In autumn last, I spent two
months in calculation to no purpose for want of a good method,
wdiich made me afterward return to the first book, and enlarge it
with diverse propositions, som£ relating to comets, others to other
tilings found ou* last winter. The third I now design to sup¬
press. Philosophy is such an impertinently litigious lady, that a
man had as good be engaged in law-suits as have to do with her.
I found it so formerly, and now I can no sooner come near her
again, but she gives me warning. The first two books without
the third will not so well bear the title of Philosophize Naturalis

3

32

LIFE OF SIR ISAAC NEWTON.

Principia Mathematicia ; and thereupon I had altered it to this,
De Motu Corporum Libri duo. But after second thought I re¬
tain the former title. It will help the sale of the book, which J
ought not to diminish now ’tis yours.”

This “ warning” arose from some pretensions put forth by Dr.
Hooke. And though Newton gave a minute and positive refuta¬
tions of such claims, yet, to reconcile all differences, he gener¬
ously added to Prop. IV. Cor. 6, Book I., a Scholium, in which
Wren, Hooke and Halley are acknowledged to have indepen¬
dently deduced the law of gravity from the second law of
Kepler.

The suppression of the third book Dr. Halley could not endure
to see. “ I must again beg you” says he, “ not to let your re¬
sentments run so high as to deprive us of your third book, where¬
in your applications of your mathematical doctrine to the theory
of comets, and several curious experiments, which, as I guess by
what you write ought to compose it, will undoubtedly render it
acceptable to those who* will call themselves philosophers without
mathematics, which are much the greater number” To these
solicitations Newton yielded. There were no “ resentments,” how¬
ever, as we conceive, in his “ design to suppress.” He sought
peace ; for he loved and valued it above all applause. But, in
spite of his efforts for tranquillity’s sake, his course of discovery
was all along molested by ignorance or presumptuous rivalry.

The publication of the great work now went rapidly forwards.
The second book was sent to the Society, and presented on the
2d March ; the third, on the 6th April; and the whole was com¬
pleted and published in the month of May, 1686-7. In the sec¬
ond Lemma of the second book, the fundamental principle of his
fluxionary calculus was, for the first time, given to the world; but
its algorithm or notation did not appear till published in the
second volume nf Dr. Wallis’s works, in 1693.

And thus was ushered into existence The Principia —a work
to which pre-eminence above all the productions of the human
intellect has been awarded—a work that must be esteemed of
priceless worth so long as Science has a votary, or a single wor¬
shipper be left to kneel at the altar of Truth.

LIFE OF SIR ISAAC NEWTON.

33

The entire work bears the general title of The Mathematical
Principles of Natural Philosophy. It consists of three books:
the first two, entitled, Of the Motion of Bodies, are occupied
with the laws and conditions of motions and forces, and are illus¬
trated with many scholia treating of some of the most general
and best established points in philosophy, such as the density and
resistance of bodies, spaces void of matter, and the motion of
sound and light. From these principles, there is deduced, in the
third book, draw T n up in as popular a style as possible and entitled,
Of the System of the World, the constitution of the system of
ihe world. In regard to this book, the author say^ —“ I had, indeed,
composed the third Book in a popular method, that it might be read
by many; but afterwards, considering that such as had not suf-
ficently entered into the principles could not easily discover the
strength of the consequences, nor lay aside the prejudices to which
they had been many years accustomed, therefore, to prevent dis¬
putes which might be raised upon such accounts, I chose to reduce
the substance of this Book into the form of Propositions (in the
mathematical way), which should be read by those only who had
first made themselves masters of the principles established in the
preceding Books : not that I would advise any one to the previous
study of every Proposition of those Books.”—“ It is enough it
one carefully reads the Definitions, the Laws of Motion, and the
three first Sections of the first Book. He may then pass on to
this Book, and consult such of the remaining Propositions of the
first two Books, as the references in this, and his occasions shall re¬
quire.” So that “ The System of the World” is composed both
“ in a popular method,” and in the form of mathematical Propo¬
sitions.

The principle of Universal Gravi 4 ition, namely, that every
particle of matter is attracted by , or gravitates to , every other
particle of matter, with a force inversely proportional to the
squares of their distances —is the discovery wl ich characterizes
The Principia. This principle the author deduced from the mo¬
tion of the moon, and the three laws of Kepler—laws, which
Newton, in turn, by his greater law, demonstrated to be true.

From the first law of Kepler, namely, the proportionality of

34

LIFE OF SIR ISAAC NEWTON.

the areas to t\ie times of their description, our author inferred
that the force which retained the planet in its orbit was always
directed to the sun; and from the second, namely, that every
planet moves in an ellipse with the sun in one of its foci, he drew
the more general inference that the force by which the planet
moves round that focus varies inversely as the square of its dis¬
tance therefrom : and he demonstrated that a planet acted upon
by such a force could not move in any other curve than a conic
section; showing when the moving body would describe a circu¬
lar, an elliptical, a parabolic, or hyperbolic orbit. He demon¬
strated, too, that this force, or attracting, gravitating power re¬
sided in every, the least particle; but that, in spherical masses, it
operated as if confined to their centres; so that, one sphere or
body will act upon another sphere or body, with a force directly
proportional to the quantity of matter, and inversely as the square
of the distance between their centres ; and that their velocities of
mutual approach will be in the inverse ratio of their quantities o f
matter. Thus he grandly outlined the Universal Law. Verify¬
ing its truth by the motions of terrestrial bodies, then by those of
the moon and other secondary orbs, he finally embraced, in one
mighty generalization, the entire Solar System—all the. move¬
ments of all its bodies—planets, satellites and comets—explain¬
ing and harmonizing the many diverse and theretofore inexplica¬
ble phenomena.

Guided by the genius of Newton, we see sphere bound to
sphere, body to body, particle to particle, atom to mass, the min¬
utest part to the stupendous whole—each to each, each to all,
and all to each—in the mysterious bonds of a ceaseless, recipro¬
cal influence. An influence whose workings are shown to be
alike present in the globular dew-drop, or oblate-spheroidal earth ;
in the falling shower, or vast heaving ocean tides; in the flying
thistle-down, or fixed, ponderous rock ; in the swinging pendulum,
or time-measuring sun; in the varying and unequal moon, or
earth’s slowly retrograding poles ; in the uncertain meteor, or
olazing comet wheeling swiftly away on its remote, yet determined
round. An influence, in fine, that may link system to system
through all the star-glowing firmament; then firmament to lirma-

LIFE OF SIR ISAAC NEWTON.

35

merit; aye, firmament to firmament, again and again, till, con¬
verging home, it may be, to some ineffable centre, where more
presently dwells He who inhabiteth immensity, and where infini¬
tudes meet and eternities have their conliux, and where around
move, in softest, swiftest measure, all the countless hosts that
crowd heaven’s fathomless deeps.

And yet Newton, amid the loveliness and magnitude of Om¬
nipotence, lost not sight of the Almighty One. A secondary,
however universal, was not taken for the First Cause. An im¬
pressed force, however diffused and powerful, assumed not the
functions of the creating, giving Energy. Material beauties,
splendours, and sublimities, however rich in glory, and endless in
extent, concealed not the attributes of an intelligent Supreme.
From the depths of his own soul, through reason and the Word,
he had risen, d 'priori , to God : from the heights of Omnipotence,
through the design and law of the budded universe, he proved a
posteriori , a Deity. “ I had,” says he, “ an eye upon such prin¬
ciples as might work, with considering men, for the belief of a
Deity,” in writing the Principia ; at the conclusion whereof, he
teaches that—“ this most beautiful system of the sun, planets and
comets, could only proceed from the counsel and dominion of an
intelligent and powerful Being. And if the fixed stars are the
centres of other like systems, these, being forme 1 by the like
wise counsels, must be all subject to the dominion of One; especially
since the light of the fixed stars is of the same nature with the
light of the sun, and from every system light passes into all other
systems : and lest the systems of the fixed stars should, by their
gravity, fall on each other mutually, he hath placed those systems
at immense distances one from another.

“ This Being governs all things, not as the soul of the world,
but as Lord over all; and on account of his dominion he is wont. #
to be called Lord God iravTongar^g or Universal Ruler ; for God
is a relative word, and has a respect to servants ; and Deity is
the dominion of God, not over his own body, as those imagine
who fancy God to be the soul of the world, but over servants.
The Supreme God is a Being eternal, infinite, absolutely perfect;
but a being, however perfect, without dominion, cannot be said to

36

LIFE OF SIR ISAAC NEWTON.

be Lord God; for we say, my God, your God, the God of Israel
the God of Gods, and Lord of Lords ; but we do not say, my
Eternal, your Eternal, the Eternal of Israel, the Eternal of Gods ;
we do not say my Infinite, or my Perfect: these are titles which
have no respect to servants. The word God usually signifies
Lord ; but every Lord is not God. It is the dominion of a spir¬
itual Being which constitutes a God ; a true, supreme, or imagi¬
nary dominion makes a true, supreme, or imaginary God. And
from his true dominion it follows that the true God is a living,
intelligent and powerful Being; and from his other perfections,
that he is supreme or most perfect. He is eternal and in¬
finite, omnipotent and omniscient; that is, his duration reaches
from eternity to eternity ; his presence from infinity to infinity;
he governs all things and knows all things, that are or can be
done. He is not eternity or infinity, but eternal and infinite ;
he is not duration and space, but he endures and is present.
He endures forever and is everywhere present; and by existing
always and everywhere, he constitutes duration and space. Since
every particle of space is always , and every indivisible moment
of duration is everywhere , certainly the Maker and Lord of things
cannot be never and nowhere. Every soul that has perception
is, though in different times and different organs of sense and mo¬
tion, still the same indivisible person. There are given succes¬
sive parts in duration, co-existent parts in space, but neither the
one nor the other in the person of a man, or his thinking
principle; and much less can they be found in the thinking sub¬
stance of God. Every man, so far as he is a thing that has per¬
ception, is one and the same man during his whole life, in all and
each of his organs of sense. God is one and the same God, al¬
ways and everywhere. He is omnipresent, not virtually only,
but also substantially; for virtue cannot subsist without sub¬
stance. In him are all things contained and moved; yet neither
affects the other ; God suffers nothing from the motion of bodies ;
bodies find no resistance from the omnipresence of God. It is
allowed by all that the Supreme God exists necessarily; and by
the same necessity he exists always and everywhere. Whence
also he is all similar, all eye, all ear, all brain, all arm, all powei

LIKE OF SIR ISAAC NEWTON.

3 ?

to perceive, to understand, and to act; but in a manner not at all
human, in a manner not at all corporeal, in a manner utterly un¬
known to us. As a blind man has no idea of colours, so have we
no idea of the manner by which the all-wise God perceives and
understands all things. He is utterly void of all body, and bodily
ligure, and can therefore neither be seen, nor heard, nor touched:
nor ought he to be worshipped under the representation of any
corporeal thing. We have ideas of his attributes, but what the
real substance of anything is we know not. In bodies we see
only their figures and colours, we hear only the sounds, we touch
only their outward surfaces, we smell only the smells, and taste
only the savours ; but their inward substances are not to be known,
either by our senses, or by any reflex act of our minds : much
less, then, have we any idea of the substance of God. We know
him only by his most wise and excellent contrivances of things,
and final causes ; we admire him for his perfections ; but we rev
erence and adore him on account of his dominion; for we adore
him as his servants ; and a god without dominion, providence, and
final causes, is nothing else but Fate and Nature. Blind meta¬
physical necessity, which is certainly the same always and every¬
where, could produce no variety of things. All that diversity of
natural things which we find suited to different times and places
could arise from nothing but the ideas and will of a Being neces¬
sarily existing.”

Thus, the diligent student of science, the earnest seeker of
truth, led, as through the courts of a sacred Temple, wherein, at
each step, new wonders meet the eye, till, as a crowning grace,
they stand before a Holy of Holies, and learn that all science and
all truth are one which hath its beginning and its end in the
knowledge of Him whose glory the heavens declare, and whose
handiwork the firmament showeth forth.

The introduction of the pure and lofty doctrines of the Prin-
cipia was perseveringly resisted. Descartes,with his system of
vortices, had sown plausibly to the imagination, and error had
struck down deeply, and shot up luxuriantly, not only in the
popular, but in the scientific mind. Besides the idea—in itself so
simple and so grand—that the great masses of the planets were

38

LIFE OF SIR ISAAC NEWTON.

suspended in empty space, and retained in their orbits by an in¬
visible influence residing in the sun—was to the ignorant a thing
inconceivable, and to the learned a revival of the occult qualities
of the ancient physics. This remark applies particularly to the
continent. Leibnitz misapprehended ; Huygens in part rejected;
John Bernoulli opposed ; and Fontenelle never received the doc¬
trines of the Principia. So that, the saying of Voltaire is prob¬
ably true, that though Newton survived the publication of his
great work more than forty years, yet, at the time of his death,
he had not above twenty followers out of England.

But in England, the reception of our author’s philosophy was
rapid and triumphant. His own labours, while Lucasian Pro¬
fessor ; those of his successors in that Chair—Whiston and
Saunderson ; those of Dr. Samuel Clarke, Dr. Laughton, Roger
Cotes, and Dr. Bentley ; the experimental lectures of Dr. Keill
and Desaguliers ; the early and powerful exertions of David
Gregory at Edinburgh, and of his brother James Gregory at St.
Andrew’s, tended to diffuse widely in England and Scotland a
knowledge of, and taste for the truths of the Principia. Indeed,
its mathematical doctrines constituted, from the first, a regular
part of academical instruction; while its physical truths, given to
the public in popular lectures, illustrated by experiments, had,
before the lapse of twenty ) (ars, become familiar to, and adopted
by the general mind. Pemberton’s popular “ View of Sir Isaac
Newton’s Philosophy” was published, in 1728 ; and the year after¬
ward, an English translation of the Principia, and System of the
World, by Andrew Motte. And since that period, the labours of
Le Seur and Jacquier, of Thorpe, of Jebb, of Wright and others
have greatly contributed to display the most hidden treasures of
the Principia.

About the time of the publication of the Principia, Janies II.,
bent on re-establishing the Romish Faith, had, among other ille¬
gal acts, ordered by mandamus, the University of Cambridge to
confer the degree of Master of Arts upon an ignorant monk.
Obedience to this mandate was resolutely refused. Newton was
one of the nine delegates chosen to defend the independence of
the University. They appeared before the High Court;—and

LIFE OF SIR ISAAC NEWTON.

39

successfully: the king abandoned his design. The prominent
part which our author took in these proceedings, and his eminence
in the scientific world, induced his proposal as one of the parlia¬
mentary representatives of the University. He was elected, in
1688, and sat in the Convention Parliament till its dissolution.
After the first year, however, he seems to have given little or no
attention to his parliamentary duties, being seldom absent from
the University till his appointment in the Mint, in 1695.

Newton began his theological researches sometime previous to
1691 ; in the prime of his years, and in the matured vigour of
his intellectual powers. From his youth, as we have seen, he
had devoted himself with an activity the most unceasing, and an
energy almost superhuman to the discovery of physical truth;—
giving to Philosophy a new foundation, and to Science a new
temple. To pass on, then, from the consideration of the material,
more directly to that of the spiritual, was a natural, nay, with so
large and devout a soul, a necessary advance. The Bible was to
him of inestimable worth. In the elastic freedom, which a pure
and unswerving faith in Him of Nazareth gives, his mighty facul¬
ties enjoyed the only completest scope for development. His
original endowment, however great, combined with a studious
application, however profound, would never, without this libera¬
tion from the dominion of passion and sense, have enabled him to
attain to that wondrous concentration and grasp of intellect, for
which Fame has as yet assigned him no equal. Gratefully he
owned, therefore, the same Author in the Book of Nature and the
Book of Revelation. These were to him as drops of the same
unfathomable ocean ;—as outrayings of the same inner splendour ;
—as tones of the same ineffable voice ;—as segments of the same
infinite curve. With great joy he had found himself enabled to
proclaim, as an interpreter, from the hieroglyphs of Creation, the
existence of a God : and now, with greater joy, and in the fulness
of his knowledge, and in the fulness of his strength, he laboured
to make clear, from the utterances of the inspired Word, the far
mightier confirmations of a Supreme Good, in all its glorious
amplitude of Being and of Attribute ; and to bring the infallible
workings thereof plainly home to the understandings and the

40

LIFE OF SIR ISAAC NEWTON.

affections of his fellow-men; and finally to add the weight of his
own testimony in favour of that Religion, whose truth is now, in¬
deed, “ girded with the iron and the rock of a ponderous and co¬
lossal demonstration.”

His work, entitled, Observations upon the Prophecies of
Holy Writ, particularly the Prophecies of Daniel and the
Apocalypse of St. John, first published in London, in 1733 4to.
consists of two parts : the one devoted to the Prophecies of
Daniel, and the other to the Apocalypse of St. John. In the first
part, he treats concerning the compilers of the books of the Old
Testament;—of the prophetic language ;—of the vision of the
four beasts;—of the kingdoms represented by the feet of the
image composed of iron and clay ;—of the ten kingdoms repre¬
sented by the ten horns of the beast;—of the eleventh horn of
Daniel’s fourth beast; of the power which should change times
and laws of the kingdoms represented in Daniel by the ram
and he-goat;—of the prophecy of the seventy weeks ;—of the
times of the birth and passion of Christ;—of the prophecy of the
Scripture of Truth ;—of the king who doeth according to his will,
and magnified himself above every god, and honoured Mahuzzims,
and regarded not the desire of women;—of the Mahuzzim, hon¬
oured by the king who doeth according to his will. In the sec¬
ond part, he treats of the time when the Apocalypse was written ,
of the scene of the vision, and the relation which the Apocalypse
has to the book of the law of Moses, and to the worship of God
in the temple ;—of the relation which the Apocalypse has to the
prophecies of Daniel, and of the subject of the prophecy itself.
Newton regards the prophecies as given, not for the gratification
of man’s curiosity, by enabling him to foreknow ; but for his con¬
viction that the world is governed by Providence, by witnessing
their fulfilment. Enough of prophecy, he thinks, has already
been fulfilled to afford the diligent seeker abundant evidence of
God’s providence. The whole work is marked by profound
erudition, sagacity and argument.

And not less learning, penetration and masterly reasoning are
conspicuous in his Historical Account of Two Notable
Corruptions of Scriptures in a Letter to a Friend. This

LIFE OF SIR ISAAC NEWTON.

41

Treatise, first accurately published in Dr. Horsley’s edition of his
works, relates to two texts: the one, 1 Epistle of St. John v. 7;
the other, 1 Epistle of St. Paul to Timothy iii. 16. As this
work had the effect to deprive the advocates of the doctrine of
the Trinity of two leading texts, Newton has been looked upon
as an Arian ; but there is absolutely nothing in his writings to
warrant such a conclusion.

His remaining theological works consist of the Lexicon Pro-
pheticum, which was left incomplete ; a Latin Dissertation on
the sacred cubit of the Jews, which was translated into English,
and published, in 1737, among the Miscellaneous Works of John
Greaves ; and Four Letters addressed to Dr. Bentley , contain -
ing some arguments in pi’oof of a Deity. These Letters were
dated respectively: 10th December, 1692; 17th January, 1693;
25th February, 1693; and 11th February, 1693—the fourth
bearing an earlier date than the third. The best faculties and
the profoundest acquirements of our author are convincingly
manifest in these lucid and powerful compositions. They were
published in 1756, and reviewed by Dr. Samuel Johnson.

Newton’s religious writings are distinguished by their absolute
freedom from prejudice. Everywhere, throughout them, there
glows the genuine nobleness of soul. To his whole life, indeed,
we may here fitly extend the same observation. He was most
richly imbued with the very spirit of the Scriptures which he so
delighted to study and to meditate upon. His was a piety, so
fervent, so sincere and practical, that it rose up like a holy incense
from every thought and act. His a benevolence that not only
willed, but endeavoured the best for all. His a philanthropy
that held in the embracings of its love every brother-man.
His a toleration of the largest and the truest; condemning per¬
secution in every, even its mildest form; and kindly encouraging
each striving after excellence :—i toleration that came not of
indifference—for the immoral and the impious met with their
quick rebuke—but a toleration that came of the wise humbleness
and the Christian charity, which see, in the nothingness of self
and the almightiness of Truth, no praise for the ablest, and no
blame for th^ feeblest in their strugglings upward to light and life.

42

LIFE OF SIR ISAAC NEWTON,

In the winter of 1691-2, on returning from chapel, one morn¬
ing, Newton found mat a favourite little dog, called Diamond,
had overturned a lighted taper on his desk, and that several pa¬
pers containing the results of certain optical experiments, were
nearly consumed. His only exclamation, on perceiving his loss,
was, “ Oh Diamond, Diamond, little knowest thou the mischiei
thou hast done,” Dr. Brewster, in his life of our author, gives the
following extract from the manuscript Diary of Mr. Abraham De
La Pryme, a student in the University at the time of this oc¬
currence.

“ 1692. February, 3.—What I heard to-day I must relate.
There is one Mr. Newton (whom I have very oft seen), Fellow
of Trinity College, that is mighty famous for his learning, being a
most excellent mathematician, philosopher, divine, &c. He has
been Fellow of the Royal Society these many years ; and among
other very learned books and tracts, he's written one upon the mathe¬
matical principles of philosophy, which has given him a mighty
name, he having received, especially from Scotland, abundance of
congratulatory letters for the same; but of all the books he ever
wrote, there was one of colours and light, established upon thou¬
sands of experiments which he had been twenty years of making,
and which had cost him many hundreds of pounds. This book
which he vaiued so much, and which was so much talked of, had
the ill luck to perish, and be utterly lost just when the learned
author was almost at pitting a conclusion at the same, after this
manner: In a winter’s morning, leaving it among his other papers
on his study table while he went to chapel, the candle, which he
had unfortunately left burning there, too, catched hold by some
means of other papers, and they fired the aforesaid book, and ut¬
terly consumed it and several other valuable writings ; and which
is most wonderful did no further mischief. But when Mr. New¬
ton came from chapel, and had seen what was done, every one
thought he would have run mad, he was so troubled thereat that
he was not himself for a month after. A long account of this his
system of colours you may find in the Transactions of the Royal
Society, which he had sent up to them long before this sad mis¬
chance happened unto him.”

LIFE OF SIR ISAAC NEWTON.

43

It will be borne in mind that all of Newton’s theological wri¬
tings, with the exception of the Letters to Dr. Bentley, were
composed before this event which, we must conclude, from
Pryme’s words, produced a serious impression upon our author for
about a month. But M. Biot, in his Life of Newton, relying on a
memorandum contained in a small manuscript Journal of Huygens,
declares this occurrence to have caused a derangement of New¬
ton’s intellect. M. Biot’s opinions and deductions, however, as
well as those of La Place, upon this subject, were based upon
erroneous data, and have been overthrown by the clearest proof.
There is not, in fact, the least evidence that Newton’s reason was,
for a single moment, dethroned; on the contrary, the testimony
is conclusive that he was, at all times, perfectly capable of carry¬
ing on his mathematical, metaphysical and astronomical inquiries.
Loss of sleep, loss of appetite, and irritated nerves will disturb
somewhat the equanimity of the most serene ; and an act done, or
language employed, under such temporary discomposure, is not a
just criterion of the general tone and strength of a man’s mind.
As to the accident itself, we may suppose, whatever might have
been its precise nature, that it greatly distressed him, and, still
further, that its shock may have originated the train of nervous
derangements, which afflicted him, more or less, for two years
afterward. Yet, during this very period of ill health, we find him
putting forth his highest powers. In 1692, he prepared for, and
transmitted to Dr. Wallis the first proposition of the Treatise on
Quadratures, with examples of it in first, second and third flux¬
ions. He investigated, in the same year, the subject of haloes;
making and recording numerous and important observations rela¬
tive thereto. Those profound and beautiful Letters to Dr. Bentley
were written at the close of this and the beginning of the next
year. In October, 1693, Locke, who was then about publishing a
second edition of his work on the Human Understanding, request¬
ed Newton to reconsider his opinions on innate ideas. And in
1694, he was zealously occupied in perfecting his lunar theory ;
visiting Flamstead, at the Koyal Observatory of Greenwich, in
September, and obtaining a series of lunar observations ; and

14

LIFE OF SIR ISAAC NEWTON.

commencing, in October, a correspondence with that distinguished
practical Astronomer, which continued till 1698.

We now arrive at the period when Newton permanently with¬
drew from the seclusion of a collegiate, and entered upon a more
active and public life. He was appointed Warden of the Mint,
in 1695, through the influence of Charles Montague, Chancellor
of the Exchequer, and afterward Earl of Halifax. The current
roin of the nation had been adulterated and debased, and Mon¬
tague undertook a re-coinage. Our author’s mathematical and
chemical knowledge proved eminently useful in accomplishing
this difficult and most salutary reform. In 1699, he was pro¬
moted to the Mastership of the Mint—an office worth twelve or
fifteen hundred pounds per annum, and which he held during the
remainder of his life. He wrote, in this capacity, an official Re¬
port on the Coinage, which has been published: he also prepared
a Table of Assays of Foreign Coins, which was printed at the
end of Hr. Arbuthnot’s Tables of Ancient Coins, Weights, and
Measures, in 1727.

Newton retained his Professorship at Cambridge till 1703.
But he had, on receiving the appointment of Master of the Mint,
in 1699, made Mr. Whiston his deputy, with all the emoluments
of the office ; and, on finally resigning, procured his nomination to
the vacant Chair.

In January 1697, John Bernouilli proposed to the most distin¬
guished mathematicians of Europe two problems for solution.
Leibnitz, admiring the beauty of one of them, requested the time
for solving it to be extended to twelve months—twice the period
originally named. The delay was readily granted. Newton, how¬
ever, sent in, the day after he received the problems, a solution of
them to the President of the Royal Society. Bernouilli obtained
solutions from Newton, Leibinitz and the Marquis De L’Hopital;
but Newton’s though anonymous, he immediately recognised
“ tanquam ungue leonem ,” as the lion is known by his claw.
We may mention here the famous problem of the trajectories
proposed by Leibnitz, in 1716, for the purpose of “ feeling the
pulse of the English Analysts.” Newton received the problem
about five o’clock in the afternoon, as he was returning from the

LIFE OF SIR ISAAC NEWTON.

45

Mint; and though it was extremely difficult and he himself much
fatigued, yet he completed its solution, the same evening before
he went to bed.

The history of these problems affords, by direct comparison, a
striking illustration of Newton’s vast superiority of mind. That
amazing concentration and grasp of intellect, of which we have
spoken, enabled him to master speedily, and, as it were, by a
single effort, those things, for the achievement of which, the many
would essay utterly in vain, and the very, very few attain only
after long and renewed striving. And yet, with a modesty as
unparalleled as his power, he attributed his successes, not to any
extraordinary sagacity, but solely to industry and patient thought.
He kept the subject of consideration constantly before him, and
waited till the first dawning opened gradually into a full and
clear light; never quitting, if possible, the mental process till the
object of it were wholly gained. He never allowed this habit of
meditation to appear in his intercourse with society; but in the
privacy of his own chamber, or in the midst of his own family, he
gave himself up to the deepest abstraction. Occupied with some
interesting investigation, he would often sit down on his bedside,
after he rose, and remain there, for hours, partially dressed.
Meal-time would frequently come and pass unheeded; so that,
unless urgently reminded, he would neglect to take the re¬
quisite quantity of nourishment. But notwithstanding his anx¬
iety to be left undisturbed, he would, when occasion required,
turn aside his thoughts, though bent upon the most intricate re¬
search, and then, when leisure served, again direct them to the
very point where they ceased to act: and this he seemed to ac¬
complish not so much by the force of his memory, as by the force
of his inventive faculty, before the vigorous intensity of which, no
subject, however abstruse, remained long unexplored.

He was elected a member of the Royal Academy of Sciences
at Paris, in 1699, when that distinguished Body were empowered,
by a new charter, to admit a small number of foreign associates.
In 1700, he communicated to Dr. Halley a description of his re¬
flecting instrument for observing the moon’s distance from the
fixed stars. This description was published in the Philosophical

46

LIFE OF SIR ISAAC NEWTON.

Transactions, in 1742. The instrument was the same as that
produced by Mr. Hadley, in 1731, and which, under the name of
Hadley’s Quadrant, has been of so great use in navigation. On
(he assembling of the new Parliament, in 1701, Newton was re¬
elected one of the members for the University of Cambridge. In
1703, he was chosen President of the Royal Society of London,
to which office he was annually re-elected till the period of his
decease—about twenty-five years afterward.

Our author unquestionably devoted more labour to, and, in
many respects, took a greater pride in his Optical, than his other
discoveries. This science he had placed on a new and indestruc¬
tible basis; and he wished not only to build, but to perfect the
costly and glowing structure. He had communicated, before the
publication of the Principia, his most important researches on
light to the Royal Society, in detached papers which were inserted
in successive numbers of the Transactions; but he did not pub¬
lish a connected view of these labours till 1704, when they appeared
under the title of Optics : or, a Treatise on the Reflexions,
Refractions, Inflexions and Colours of Light. To this,
but to no subsequent edition, were added two Mathematical Trea¬
tises, entitled, Tractatus duo de speciebus et magnitudine
figurarum curvilinearum ; the one bearing the title Tractatus
de Quadratura Curvarum; and the other, that of Enumeratio
linearum tertii ordinis. The publication of these Mathemati¬
cal Treatises was made necessary in consequence of plagiarisms
from the manuscripts of them loaned by the author to his friends.
Dr. Samuel Clarke published a Latin translation of the Optics, in
in 1706 ; whereupon he was presented by Newton, as a mark of
his grateful approbation, with five hundred pounds, or one hun¬
dred pounds for each of his children. The work was afterward -
translated into French. It had a remarkably wide circulation,
and appeared, in several successive editions, both in England and
on the Continent. There is displayed, particularly on this Opti¬
cal Treatise, the author’s talent for simplifying and communica¬
ting the profoundest speculations. It is a faculty rarely united to
that of the highest invention. Newton possessed both ; and thus
that mental perfectness which enabled him to create, to combine,

LIFE OF SIR ISAAC NEWTON.

47

and to teach, and so render himself, not the “ornament” cnly,
but inconceivably more, the pre-eminent benefactor of his species.

The honour of knighthood was conferred on our author in
1705. Soon afterward, he was a candidate again for the Repre¬
sentation of the University, but was defeated by a large majority.
It is thought that a more pliant man was preferred by both min¬
isters and electors. Newton was always remarkable for simplicity
of dress, and his only known departure from it was on this oc¬
casion, when he is said to have appeared in a suit of laced
clothes.

The Algebraical Lectures which he had, during nine years,
delivered at Cambridge, were published by Whiston, in 1707,
under the title of Arithmetica Universalis, sine de Composi¬
tions et Resolutione Arithmetica Liber. This publication
is said to have been a breach of confidence on Whiston’s part. Mr.
Ralphson, not long afterward, translated the work into English;
and a second edition of it, with improvements by the author, was
issued at London, 1712, by Dr. Machin. Subsequent editions,
both in English and Latin, with commentaries, have been published.

In June, 1709, Newton intrusted the superintendence of a
second edition of the Principia to Roger Cotes, Plumian Pro¬
fessor of Astronomy at Cambridge. The first edition had been
sold off for some time. Copies of the work had become very
rare, and could only be obtained at several times their original
cost. A great number of letters passed oetween the author and
Mr. Cotes during the preparation of the edition, which finally
appeared in May, 1713. It had many alterations and improve¬
ments, and was accompanied by an admirable Preface from the
pen of Cotes.

Our author’s early Treatise, entitled, Analysis per Equationes
Numero Terminorum Infinitas, as well as a small Tract, oearing
the title of Methodus Differentialis, was published, wifn ms
consent, in 1711. The r former of these, and the Treatise De
Quadratura Curvarum, translated into Englisn, witn a *arge com¬
mentary, appeared in 1745. His work, entitled. Artis Ana¬
lytic.® Specimina, vel Geometria Analytica, was nrs; given
to the world in the edition of Dr. Horsley, 1779.

4

48

LIFE OF SIR ISAAC NEWTON.

It is a notable fact, in Newton’s history, that he never volun*
tarily published any one of his purely mathematical writings
The cause of this unwillingness in some, and, in other instances,
of his indifference, or, at least, want of solicitude to put forth his
works may be confidently sought for in his repugnance to every¬
thing like contest or dispute. But, going deeper than this aver¬
sion, we find, underlying his whole character and running parallel
with all his discoveries, that extraordinary humility which always
preserved him in a position so relatively just to the behests of
time and eternity, that the infinite value of truth, and the utter
worthlessness of fame, were alike constantly present to him.
Judging of his course, however, in its more temporary aspect, as
bearing upon his immediate quiet, it seemed the most unfortunate.
For an early publication, especially in the case of his Method of
Fluxions, would have anticipated all rivalry, and secured him
from the contentious claims of Leibnitz. Still each one will solve
the problem of his existence in his own way, and, with a manlike
Newton, his own, as we conceive, could be no other than the best
way. The conduct of Leibnitz in this affair is quite irreconcilable
with the stature and strength of the man; giant-like, and doing
nobly, in many ways, a giant’s work, yet cringing himself into the
dimensions and performances of a common calumniator. Opening
in 1699, the discussion in question continued till the close of
Leibnitz’s life, in 1716. We give the summary of the case as
contained in the Report of the Committee of the Royal Society,
the deliberately weighed opinion of which has been adopted as an
authoritative decision in all countries.

“ We have consulted the letters and letter books in the custody
of the Royal Society, and those found among the papers of Mr.
John Collins, dated between the years 1669 and 1677, inclusive;
and showed them to such as knew and avouched the hands of Mr.
Barrow, Mr. Collins, Mr. Oldenburg, and Mr. Leibnitz; and
compared those of Mr. Gregory with one another, and with copies
of some of them taken in the hand of Mr. Collins ; and have
extracted from them what relates to the matter referred to us;
all which extracts, herewith delivered to you, we believe to be
genuine and authentic. And by these letters and papers wp
find:—

LIFE OF SIR ISAAC NEWTON.

49

“ I. Mr. Leibnitz was in London in the beginning of the year
1673 ; and went thence in or about March, to Paris, where he
kept a correspondence with Mr. Collins, by means of Mr. Olden¬
burg, till about September, 1676, and then returned, by London
and Amsterdam, to Hanover: and that Mr. Collins was very free
in communicating to able mathematicians what he had received
from Mr. Newton and Mr. Gregory.

“ II. That when Mr. Leibnitz was the first time in London,
he contended for the invention of another differential method,
properly so called; and, notwithstanding he was shown by Dr.
Pell that it was Newton’s method, persisted in maintaining it to
be his own invention, by reason that he had found it by himself
without knowing what Newton had done before, and had much
improved it. And we find no mention of his having any other
differential method than Newton’s before his letter of the 21st of
June, 1677, which was a year after a copy of Mr. Newton’s letter
of the 10th of December, 1672, had been sent to Paris to be
communicated to him; and above four years after Mr. Collins
began to communicate that letter to his correspondents ; in which
letter the method of fluxions was sufficiently described to any
intelligent person.

“III. That by Mr. Newton’s letter, of the 13th of June, 1676
it appears that he had the method of fluxions above five years
before the writing of that letter. And by his Analysis per AEqua-
tiones numero Terminorum Infinitas, communicated by Dr. Barrow
to Mr. Collins, in July, 1669, we find that he had invented the
method before that time.

“IV. That the differential method is one and the same with
the method of fluxions, excepting the name and mode of notation ;
Mr. Leibnitz calling those quantities differences wffiich Mr. Newton
calls moments, or fluxions; and marking them with a letter d —a
mark not used by Mr. Newton.

“ And, therefore, we take the proper question to be, not who
invented this or that method, but, who was the first inventor of
the method ? And we believe that those who have reputed Mr.
Leibnitz the first inventor knew little or nothing of his correspond¬
ence with Mr. Collins and Mr. Oldenburg long before, nor of Mr.

50

LIFE OF SIR ISAAC NEWTON.

Newton’s hiving that method above fifteen years before Mr
Leibnitz began to publish it in the Acta Eruditorum of Leipsic.

“ For which reason we reckon Mr. Newton the first inventor;
and are of opinion that Mr. Keill, in asserting the same, has been
no ways injurious to Mr. Leibnitz. And we submit to the judg¬
ment of the Society, whether the extract and papers, now pre¬
sented to you, together with what is extant, to the same pur¬
pose, in Dr. Wallis’s third volume, may not deserve to be made
public.”

This Report, with the collection of letters and manuscripts,
under the title of Commercium Epistolicum D. Johannis Collins

ET ALIORUM DE ANALYSI PROMOTA JuSSU SoCIETATlS REGIES

Editum, appeared accordingly in the early part of 1713. Its
publication seemed to infuse additional bitterness into the feelings
of Leibnitz, who descended to unfounded charges and empty
threats. He had been privy counsellor to the Elector of Han¬
over, before that prince was elevated to the British throne; and
in his correspondence, in 1715 and 1716, with the Abbe Conti,
then at the court of George L, and with Caroline, Princess of
Wales, he attacked the doctrines of the Principia, and indirectly
its author, in a manner very discreditable to himself, both as a
learned and as an honourable man. His assaults, however, were
triumphantly met; and, to the complete overthrow of his rival
pretensions, Newton was induced to give the finishing blow. The
verdict is universal and irreversible that the English preceded
the German philosopher, by at least ten years, in the invention
of fluxions. Newton could not have borrowed from Leibnitz;
but Leibnitz might have borrowed from Newton. Anew edition
of the Commercium Epistolicum was published in 1722-5 (?) ; but
neither in this, nor in the former edition, did our author take any
part. The disciples, enthusiastic, capable and ready, effectually
shielded, with the buckler of Truth, the character of the Master,
whose own conduct throughout was replete with delicacy, dignity
and justice. He kept aloof from the controversy—in which Dr.
Keill stood forth as the chief representative of the Newtonian
side—till the very last, when, for the satisfaction of the King,
George I., rather than for his own, he consented to put forth his

LIFE OF SH NEWTON. 5i

hand and firmly secure his rights upon a certain and impregnable
basis.

A petition to have inventions for promoting the discovery of the
longitude at sea, suitably rewarded, was presented to the House
of Commons, in 1714. A committee, having been appointed to
investigate the subject, called upon Newton and others for their
opinions. That of our author was given in writing. A report,
favourable to the desired measure, was then taken up, and a bill
for its adoption subsequently passed.

On the ascension of George I., in 1714, Newton became an
object of profound interest at court. His position under govern¬
ment, his surpassing fame, his spotless character, and. above all,
his deep and consistent piety, attracted the reverent regard of the
Princess of Wales, afterward queen-consort to George II. She
was a woman of a highly cultivated mind, and derived the greatest
pleasure from conversing with Newton and corresponding with
Leibnitz. One day, in conversation with her, our author men¬
tioned and explained a new system of chronology, which he had
composed at Cambridge, where he had been in the habit “ of
refreshing himself with history and chronology, when he wa°
weary with other studies.” Subsequently, in the year 1718, she
requested a copy of this interesting and ingenious work. Newton,
accordingly, drew up an abstract of the system from the separate
papers in which it existed, and gave it to her on condition that it
should not be communicated to any other person. Sometime
afterward she requested that the Abbe Conti might be allowed
to have a copy of it. The author consented: and the abbe
received a copy of the manuscript, under the like injunction and
promise of secrecy. This manuscript bore the title of “ A short
Chronicle, from the First Memory of Things in Europe, to the
Conquest of Persia, by Alexander the Great.”

After Newton took up his residence in London, he lived in a
style suited to his elevated position and rank. He kept his car¬
riage, with an establishment of three male and three female serv¬
ants. But to everything like vain show and luxury he was utterly
averse. His household affairs, for the last twenty years of his
life, were under the charge of his niece, Mrs. Catherine Barton,

52

LIFE OF SIR ISAAC NEWTON.

wife and widow of Colonel Barton—a woman of great beauty and
accomplishment—and subsequently married to John Conduit, Esq.
At home Newton was distinguished by that dignified and gentle
hospitality which springs alone from true nobleness. On all pro¬
per occasions, he gave splendid entertainments, though without
ostentation. In society, whether of the palace or the cottage,
his manner was self-possessed and urbane; his look benign and
affable; his speech candid and modest; his whole air undisturb¬
edly serene. He had none of what are usually called the singu¬
larities of genius ; suiting himself easily to every company—
except that of the vicious and wicked; and speaking of himself
and others, naturally, so as never even to be suspected of vanity.
There was in him, if we may be allowed the expression, a whole¬
ness of nature, which did not admit of such imperfections and
weakness—the circle was too perfect, the law too constant, and
the disturbing forces too slight to suffer scarcely any of those
eccentricities which so interrupt and mar the movements of many
bright spirits, rendering their course through the world more like
that of the blazing meteor than that of the light and life-impart¬
ing sun. In brief, the words greatness and goodness could
not, humanly speaking, be more fitly employed than when applied
as the pre-eminent characteristics of this pure, meek and vene¬
rable sage.

In the eightieth year of his age, Newton was seized with
symptoms of stone in the bladder. His disease was pronounced
incurable. He succeeded, however, by means of a strict regimen,
and other precautions, in alleviating his complaint, and procuring
long intervals of ease. His diet, always frugal, was now extremely
temperate, consisting chiefly of broth, vegetables, and fruit, with,
now and then, a little butcher meat. He gave up the use of his
carriage, and employed, in its stead, when he went out, a chair.
All invitations to dinner were declined; and only small parties
were received, occasionally, at his own house.

In 1724 he wrote to the Lord Provost of Edinburgh, offering
to contribute twenty pounds yearly toward the salary of Mr.
Maclaurin, provided he accepted the assistant Professorship of
Mathematics in the University of that place. Not only in the

LIFE OF SIR ISAAC NEWTON.

53

cause of ingenuity and learning, but in that of religion—in relieving
the poor and .assisting his relations, Newton annually expended
large sums. He was generous and charitable almost to a fault.
Those, he would often remark, who gave away nothing till they
died, never gave at all. His wealth had become considerable by
a prudent economy; but he regarded money in no other light
than as one of the means wherewith he had been intrusted to do
good, and he faithfully employed it accordingly.

He experienced, in spite of all his precautionary measures, a
return of his complaint in the month of August, of the same year,
1724, when he passed a stone the size of pea ; it came from him
in two pieces, the one at the distance of two days from the other.
Tolerable good health then followed for some months. In Janu¬
ary, 1725, however, he was taken with a violent cough and inflam¬
mation of the lungs. In consequence of this attack, he was pre¬
vailed upon to remove to Kensington, where his health greatly
improved. In February following, he was attacked in both feet
with the gout, of the approach of which he had received, a few
years before, a slight warning, and the presence of which now
produced a very beneficial change in his general health. Mr.
Conduit, his nephew, has recorded a curious conversation which
took place, at or near this time, between himself and Sir Isaac.

“I was, on Sunday night, the 7th March, 1724-5, at Kensing¬
ton, with Sir Isaac Newton, in his lodgings, just after he was out
of a fit of the gout, which he had had in both of his feet, for the
first time, in the eighty-third year of his age. He was better after
it, and his head clearer and memory stronger than I had known
them for some time. He then repeated to me, by way of dis¬
course, very distinctly, though rather in answer to my queries,
than in one continued narration, what he had often hinted to me
before, viz.: that it was his conjecture (he would affirm nothing)
that there was a sort of revolution in the heavenly bodies ; that
the vapours and light, emitted by the sun, which had their sedi¬
ment, as water and other matter, had gathered themselves, by
degrees, into a body, and attracted more matter from the planets,
and at last made a secondary planet (viz.: one of those that go
round another planet), and then, by gathering to them, and

54

LIFE OF SIR ISAAC NEWTON.

attracting more matter, became a primary planet; and then, bf
increasing still, became a comet, which, after certain revolutions,
by coining nearer and nearer to the sun, had all its volatile parts
condensed, and became a matter lit to recruit and replenish the
sun (which must waste by the constant heat and light it emitted),
as a faggot would this lire if put into it (we were sitting by a
wood lire), and that that would probably be the effect of the
comet of 1680, sooner or later ; for, by the observations made
upon it, it appeared, before it came near the sun, with a tail only
two or three degrees long ; but, by the heat it contracted, in going
so near the sun, it seemed to have a tail of thirty or forty degrees
when it went from it; that he could not say when this comet
would drop into the sun; it might perhaps have live or six revo¬
lutions more first, but whenever it did it would so much increase
the heat of the sun that this earth would be burned, and no ani :
mals in it could live. That he took the three phenomena, seen
by Hipparchus, Tycho Brahe, and Kepler’s disciples, to have been
of this kind, for he could not otherwise account for an extraor¬
dinary light, as those were, appearing, all at once, among the
the fixed stars (all which he took to be suns, enlightening other
planets, as our sun does ours), as big as Mercury or Venus seems
to us, and gradually diminishing, for sixteen months, and then
sinking into nothing. He seemed to doubt whether there were
not intelligent beings, superior to us, who superintended these
revolutions of the heavenly bodies, by the direction of the Supreme
Being. He appeared also to be very clearly of opinion that the
inhabitants of this world were of short date, and alledged, as one
reason for that opinion, that all arts, as^ letters, ships, printing,
needle, &c., were discovered within the memory of history, which
could not have happened if the world had been eternal; and that
there were visible marks of ruin upon it which could not be
effected by flood only. When I asked him how this earth could
have been repeopled if ever it had undergone the same fate
it was threatened with hereafter, by the comet of 1680, he
answered, that required the power of a Creator. He said he
took all the planets to be composed of the same matter with this
earth, viz.: earth, water, stones, &c., but variously concocted. J

LIFE OF SIR ISAAC NEWTON.

55

asked him why he would not publish his conjectures, as conjec¬
tures, and instanced that Kepler had communicated his; and
though he had not gone near so far as Kepler, yet Kepler’s
guesses were so just and happy that they had been proved and
demonstrated by him. His answer was, “I do not deal in con¬
jectures.” But, on my talking to him about the four observations
that had been made of the comet of 1680, at 574 years’ distance,
and asking him the particular times, he opened his Principia ,
which laid on the table, and showed me the particular periods,
viz.: 1st. The Julium Sidus, in the time of Justinian, in 1106,
in 1680.

“ And I, observing that he said there of that comet, ‘ incidet
in corpus solis,’ and in the next paragraph adds, ‘ stellae fixae
refici possunt,’ told him I thought he owned there what we had
been talking about, viz.: that the comet would drop into the sun,
and that fixed stars were recruited and replenished by comets
when they dropped into them; and, consequently, that the sun
would be recruited too; and asked him why he would not own as
fully what he thought of the sun as well as what he thought of
the fixed stars. He said, ‘that concerned us more;’ and, laugh¬
ing, added, that he had said enough for people to know his
meaning.”

In the summer of 1725, a French translation of the chronolo¬
gical MS., of which the Abbe Conti had been permitted, some
time previous, to have a copy, was published at Paris, in violation
of all good faith. The Punic Abbe had continued true to his
promise of secrecy while he remained in England ; but no sooner
did he reach Paris than he placed the manuscript into the hands
of M. Freret, a learned antiquarian, who translated the work, and
accompanied it with an attempted refutation of the leading points
of the system. In November, of the same year, Newton received
a presentation copy of this publication, which bore the title of
Abrege de Chronologie de M. le Chevalier Newton, fait

PAR LUI-MEME, ET TRADUIT SUR LE MANUSCRIPT AnGLAIS. Soon

afterward a paper entitled, Remarks on tfe Obervations made
on a Chronological Index of Sir Isaac Newton, translated
into French by the Observator, ane published at Paris,

56

LIFE OF SIR ISAAC NEWTON,

was drawn up by our author, and printed in the Philosophical
Transactions for 1725. It contained a history of the whole
matter, and a triumphant reply to the objections of M. Freret.
This answer called into the field a fresh antagonist, Father Soueiet,
whose five dissertations on this subject were chiefiy remarkable
for the want of knowledge and want of decorum, which they
displayed. In consequence of these discussions, Newton was in¬
duced to prepare his larger work for the press, and had nearly
completed it at the time of his death. It was published in 1728,
under the title of The Chronology of the Ancient Kingdoms
Amended, to which is prefixed a short Chronicle from the

FIRST MEMORY OF THINGS IN EUROPE TO THE CONQUEST OF

Persia by Alexander the Great. It consists of six chap¬
ters: 1. On the Chronology of the Greeks; according to Whis-
ton, our author wrote out eighteen copies of this chapter with his
own hand, differing little from one another. 2. Of the Empire
of Egypt; 3. Of the Assyrian Empire ; 4. Of the two contempo¬
rary Empires of the Babylonians and Medes; 5. A Description
of the Temple of Solomon ; 6. Of the Empire of the Persians ;
this chapter was not found copied with the other five, but as it
was discovered among his papers, and appeared to be a continu¬
ation of the same work, the Editor thought proper to add it
thereto. Newton's Letter to a person of distinction who
had desired his opinion of the learned Bishop Lloyd’s
Hypothesis concerning the form of tiie most ancient
v ear, closes this enumeration of his Chronological Writings.

A third edition of the Principia appeared in 1726, with many
changes and additions. About four years were consumed in its
preparation and publication, w T hich were under the superintend¬
ance of Dr. Henry Pemberton, an accomplished mathematician,
and the author of “A view of Sir Isaac Newton’s Philo¬
sophy.” 1728. This gentleman enjoyed numerous opportunities
of conversing with the aged and illustrious author. “ I found,”
says Pemberton, “ he had read fewer of the modern mathemati¬
cians than one could have expected; but his own prodigious
invention readily supplied him with what he might have an occa¬
sion for in the pursuit of any subject he undertook. I have often

LIFE OF SIR ISAAC NEWTON.

57

heard him censure the handling geometrical subjects ly algebraic
calculations; and his book of Algebra he called by the name of
Universal Arithmetic, in opposition to the injudicious title of
Geometry, which Descartes had given to the treatise, wherein he
shows how the geometer may assist his invention by such kind
of computations. He thought Huygens the most elegant of any
mathematical writer of modern times, and the most just imitator
of the ancients. Of their taste and form of demonstration, Sir
Isaac always professed himself a great admirer. I have heard
him even censure himself for not following them yet more closely
than he did ; and speak with regret of his mistake at the begin¬
ning of his mathematical studies, in applying himself to the works
of Descartes and other algebraic writers, before he had considered
the elements of Euclid with that attention which so excellent a
writer deserves.”

“ Though his memory was much decayed,” continues Dr. Pem¬
berton, “he perfectly understood his own writings.” And even
this failure of memory, we would suggest, might have been more
apparent than real, or, in medical terms, more the result of func¬
tional weakness than organic decay. Newton seems never to
have confided largely to his memory: and as this faculty mani¬
fests the most susceptibility to cultivation ; so, in the neglect of
due exercise, it more readily and plainly shows a diminution of
its powers.

Equanimity and temperance had, indeed, preserved Newton
singularly free from all mental and bodily ailment. His hair was,
to the last, quite thick, though as white as silver. He never
made use of spectacles, and lost but one tooth to the day of his
death. He was of middle stature, well-knit, and, in the latter
part of his life, somewhat inclined to be corpulent. Mr. Conduit
says, “ he had a very lively and piercing eye, a comely and gra¬
cious aspect, with a fine head of hair, white as silver, without any
baldness, and when his peruke was off was a venerable sight.”
According to Bishop Atterbury, “in the whole air of his face and
make there was nothing of that penetrating sagacity which
appears in his compositions. He had something rather languid
in his look and manner which did not raise any great expectation

58

LIFE OF SIR ISaAC NEWTON.

in those who did not know him.” Hearne remarks, “ Sir Isaac
was a man of no very promising aspect. He was a short, well-
set man. He was full of thought, and spoke very little in com¬
pany, so that his conversation was not agreeable. When he rode
in his coach, one arm would be out of his coach on one side and
the other on the other.” These different accounts we deem
easily reconcilable. In the rooms of the Royal Society, in the
street, or in mixed assemblages, Newton’s demeanour—always
courteous, unassuming and kindly—still had in it the overawings
of a profound repose and reticency, out of which the communica¬
tive spirit, and the “lively and piercing eye” would only gleam
in the quiet and unrestrained freedom of his own fire-side.

“ But this I immediately discovered in him,” adds Pemberton,
still further, “ which at once both surprised and charmed me.
Neither his extreme great age, nor his universal reputation had
rendered him stiff in opinion, or in any degree elated. Of this I
had occasion to have almost daily experience. The remarks I
continually sent him by letters on his Principia, were received
with the utmost goodness. These were so far from being any¬
ways displeasing to him, that, on the contrary, it occasioned him
to speak many kind things of me to my friends, and to honour me
with a public testimony of his good opinion.” A modesty, open¬
ness, and generosity, peculiar to the noble and comprehensive
spirit of Newton. “ Full of w T isdom and perfect in beauty,” yet
not lifted up by pride nor corrupted by ambition. None, how¬
ever, knew so well as himself the stupendousness of his discoveries
in comparison with all that had been previously achieved; and
none realized so thoroughly as himself the littleness thereof in
comparison with the vast region still unexplored. A short time
before his death he uttered this memorable sentiment:—“ I do not
know what I may appear to the world; but to myself I seem to
have been only like a boy playing on the sea-shore, and diverting
myself in now and then finding a smoother pebble or a prettier
shell than ordinary, while the great ocean of truth lay all undis¬
covered before me.” How few ever reach the shore even, much
less find “ a smoother pebble or a prettier shell!”

Newton had now resided about two years at Kensington; and

LIFE OF SIR ISAAC NEWTON.

59

the air which he enjoyed there, and the state of absolute rest,
proved of great benefit to him. Nevertheless he would occasion¬
ally go to town. And on Tuesday, the 28th of February, 1727,
he proceeded to London, for the purpose of presiding at a meeting
of the Royal Society. At this time his health was considered,
by Mr. Conduit, better than it had been for many years. But
the unusual fatigue he was obliged to suffer, in attending the
meeting, and in paying and receiving visits, speedily produced a
violent return of the affection in the bladder. He returned to
Kensington on Saturday, the 4th of March. Dr. Mead and Dr.
Cheselden attended him; they pronounced his disease to be the
stone, and held out no hopes of recovery. On Wednesday, the
15th of March, he seemed a little better; and slight, though
groundless, encouragement was felt that he might survive the
attack. From the very first of it, his sufferings had been intense.
Paroxysm followed paroxysm, in quick succession: large drops
)f sweat rolled down his face; but not a groan, not a complaint,
not the least mark of peevishness or impatience escaped him:
and during the short intervals of relief, he even smiled and con¬
versed with his usual composure and cheerfulness. The flesh
quivered, but the heart quaked not; the impenetrable gloom was
settling down: the Destroyer near; the portals of the tomb
opening, still, amid this utter wreck and dissolution of the mortal,
the immortal remained serene, unconquerable: the radiant light
broke through the gathering darkness ; and Death yielded up its
sting, and the grave its victory. On Saturday morning, 18th,
he read the newspapers, and carried on a pretty long conversation
with Dr. Mead. His senses and faculties were then strong and
vigorous ; but at six o’clock, the same evening, he became insen¬
sible ; and in this state he continued during the whole of Sunday,
and till Monday, the 20th, when he expired, between one and
two o’clock in the morning, in the eighty-fifth year of his age.

And these were the last days of Isaac Newton. Thus closed
the career of one of earth’s greatest and best men. His mission
was fulfilled. Unto the Giver, in many-fold addition, the talents
were returned. While it was yet day he had worked; and for
the night that quickly cometh he was not unprepared. Full of

60

LIFE OF SIR ISAAC NEWTON.

years, md full of honours, the heaven-sent was recalled; and, in
the confidence of a “ certain hope,” peacefully he passed awaj
into the silent depths of Eternity.

His body was placed in Westminster Abbey, with the state
and ceremonial that usually attended the interment of the most
distinguished. In 1731, his relatives, the inheritors of his personal
estate, erected a monument to his memory in the most conspicu¬
ous part of the Abbey, which had often been refused by the dean
and chapter to the greatest of England’s nobility. During the
same year a medal was struck at the Tower in his honour; and,
in 1755, a full-length statue of him, in white marble, admirably
executed, by Roubiliac, at the expense of Dr. Robert Smith, was
erected in the ante-chamber of Trinity College, Cambridge.
There is a painting executed in the glass of one of the windows
of the same college, made pursuant to the will of Dr. Smith, who
left five hundred pounds for that purpose.

Newton left a personal estate of about thirty-two thousand
pounds. It was divided among his four nephews and four nieces
of the half blood, the grand-children of his mother, by the Reve¬
rend Mr. Smith. The family estates of Woolsthorpe and Sustern
fell to John Newton, the heir-at-law, whose great grand-father
was Sir Isaac’s uncle. Before his death he made an equitable
distribution of his two other estates: the one in Berkshire to the
sons and daughter of a brother of Mrs. Conduit; and the other,
at Kensington, to Catharine, the only daughter of Mr. Conduit,
and who afterward became Viscountess Lymington. Mr. Con¬
duit succeeded to the offices of the Mint, the duties of which he
had discharged during the last two years of Sir Isaac’s life.

Our author’s works are found in the collection of Castilion,
Berlin, 1744, 4to. 8 tom.; in Bishop Horsley’s Edition, London,
1779, 4to. 5 vol.; in the Biographia Brittannica, &c. Newton
also published Bern. Varenii Geographia, &c., 1681, 8vo.
There are, however, numerous manuscripts, letters, and other
papers, which have never been given to the world: these are
preserved, in various collections, namely, in the library of Trinity
College, Cambridge; in the library of Corpus Christi College,
Oxford; in the library of Lord Macclesfield; and, lastly and

LIFE OF SIR ISAAC NEWTON.

61

chiefly, in the possession of the family of the Earl of Portsmouth,
through the Viscountess Lymington.

Everything appertaining to Newton has been kept and che¬
rished with peculiar veneration. Different memorials of him are
preserved in Trinity College, Cambridge; in the rooms of the
Royal Society, of London ; and in the Museum of the Royal
Society of Edinburgh.

The manor-house, at Woolsthorpe, was visited by Dr. Stuke
ley, in October, 1721, who, in a letter to Dr. Mead, written in
1727, gave the following description of it:—’Tis built of stone,
as is the way of the country hereabouts, and a reasonably good
one. They led me up stairs and showed me Sir Isaac’s stud) ,
where I supposed he studied, when in the country, in his younger
days, or perhaps when he visited his mother from the University.
I observed the shelves were of his own making, being pieces of
deal boxes, which probably he sent his books and clothes down
in on those occasions. There were, some years ago, two or threr
hundred books in it of his father-in-law, Mr. Smith, which Sir
Isaac gave to Dr. Newton, of our town.” The celebrated apple-
tree, the fall of one of the apples of which is said to have turned
the attention of Newton to the subject of gravity, was destroyed
by the wind about twenty years ago; but it has been preserved
in the form of a chair. The house itself has been protected with
religious care. It was repaired in 1798, and a tablet of white
marble put up in the room where our author was born, with the
following inscription :—

“ Sir Isaac Newton, son of John Newton, Lord of the Manor
of Woolsthorpe, was born in this room, on the 25th of December,
1642”

Nature and Nature’s Laws were hid in night,

God said, “ Let Newton be,” and all was light.

THE AUTHOR’S PREFACE

Since the ancients (as we are told by Pappus), made great account of
the science of mechanics in the investigation of natural things: and the
moderns, laying aside substantial forms and occult qualities, have endeav¬
oured to subject the phenomena of nature to the laws of mathematics, I
have in this treatise cultivated mathematics so far as it regards philosophy.
I'he ancients considered mechanics in a twofold respect; as rational, which
proceeds accurately by demonstration; and practical. To practical me¬
chanics all the manual arts belong, from which mechanics took its name.
But as artificers do not work with perfect accuracy, it comes to pass that
mechanics is so distinguished from geometry, that what is perfectly accu¬
rate is called geometrical; what is less so, is called mechanical. But the
errors are not in the art, but in the artificers. He that works with less
accuracy is an imperfect mechanic; and if any could work with perfect
accuracy, he would be the most perfect mechanic of all; for the description
if right lines and circles, upon which geometry is founded, belongs to me¬
chanics. Geometry does not teach us to draw these lines, but requires
them to be drawn ; for it requires that the learner should f rst be taught
to describe these accurately, before he enters upon geometry ; then it shows
how by these operations problems may be solved. To describe right lines
and circles are problems, but not geometrical problems. The solution of
these problems is required from mechanics; and by geometry the use of
them, when so solved, is shown ; and it is the glory of geometry that from
those few principles, brought from without, it is able to produce so many
things. Therefore geometry is founded in mechanical practice, and is
nothing but that part of universal mechanics which accurately proposes
and demonstrates the art of measuring. But since the manual arts arc
chiefly conversant in the moving of bodies, it comes to pass that geometry
is commonly referred to their magnitudes, and mechanics to their motion.
In this sense rational mechanics will be the science of motions resulting
from any forces whatsoever, and of the forces required to produce any mo¬
tions, accurately proposed and demonstrated. This part of mechanics was

iXVlll

THE AUTHOR’^ PREFACE.

cultivated by the ancients in the five powers which relate to manual arts
who considered gravity (it not being a manual power), fro otherwise than
as it moved weights by those powers. Our design not respecting arts, hut
philosophy, and our subject not manual but natural powers, we consider
chiefly those things which relate to gravity, levity, elastic force, the resist¬
ance of fluids, and the like forces, whether attractive or impulsive; and
therefore we offer this work as the mathematical principles :f philosophy; for
all the difficulty of philosophy seems to consist in this—from the phenom¬
ena of motions to investigate the forces of nature, and then from these
forces to demonstrate the other phenomena; and to this end the general
propositions in the first and second book are directed. In the third book
we give an example of this in the explication of the System of the World ;
for by the propositions mathematically demonstrated in the former books,
we in the third derive from the celestial phenomena the forces of gravity
with which bodies tend to the sun and the several planets. Then from these
forces, by other propositions which are also mathematical, we deduce the mo¬
tions of the planets, the comets, the moon, and the sea. I wish we could de¬
rive the rest of the phenomena of nature by the same kind of reasoning from
mechanical principles; for I am induced by many reasons to suspect that
they may all depend upon certain forces by which the particles of bodies,
by some causes hitherto unknown, are either mutually impelled towards
each other, and cohere in regular figures, or are repelled and recede from
each other; which forces being unknown, philosophers have hitherto at¬
tempted the search of nature in vain; but I hope the principles here laid
down will afford some light either to this or some truer method of philosophy.

In the publication of this work the most acute and universally learned
Mr. Edmund Halley not only assisted me with his pains in correcting the
press and taking care of the schemes, but it was to his solicitations that its
becoming public is owing; for when he had obtained of me my demonstra¬
tions of the figure of the celestial orbits, he continually pressed me to com¬
municate the same to the Rnycd Society, who afterwards, by their kind en¬
couragement and entreaties, engaged me to think of publishing them. But
after I had begun to consider the inequalities of the lunar motions, and
had entered upon some other things relating to the laws and measures of
gravity, and other forces; and the figures that would be described by bodies
attracted according to given laws; and the motion of several bodies moving
among themselves; the motion of bodies in resisting mediums; the forces,
densities, and motions, of m< Hums; the orbits of the comets, and such like;

the author’s preface. lxix

deferred that publication till I bad made a search into those matters, and
could put forth the whole together. What relates to the lunar motions (be¬
ing imperfect), I have put all together in the corollaries of Prop. 66, to
avoid being obliged to propose and distinctly demonstrate the several things
there contained in a method more prolix than the subject deserved, and in¬
terrupt the series of the several propositions. Some things, found out after
the rest, I chose to insert in places less suitable, rather than change the
number of the propositions and the citations. I heartily beg that what 1
have here done may be read with candour; and that the defects in a
subject so difficult be not so much reprehended as kindly supplied, and in¬
vestigated by new endeavours of my readers.

ISAAC NEWTON.

Cambridge, Trinity Couege May 8, l*)8b\

In the second edition the second section of the first book was enlarged.
In the seventh section of the second book the theory of the resistances of fluids
was more accurately investigated, and confirmed by new experiments. In
the third book the moon’s theory and the praecession of the equinoxes were
more fully deduced from their principles; and the theory of the comets
was confirmed by more examples of the calculati m of their orbits, done
also with greater accuracy.

In this third edition the resistance of mediums is somewhat more largely
handled than before; and new experiments of the resistance of heavy
bodies falling in air are added. In the third book, the argument to prove
that the moon is retained in its orbit by the force of gravity is enlarged
on; and there are added new observations of Mr. Pound’s of the proportion
of the diameters of Jupiter to each other: there are, besides, added Mr.
Kirk’s observations of the comet in 16S0; the orbit of that comet com¬
puted in an ellipsis by Dr. Halley; and the ortit of the comet in 1723
computed by Mr. Bradley.

THE

MATHEMATICAL PRINCIPLES

OF

NATURAL PHILOSOPHY.

DEFINITIONS.

DEFINITION I.

The quantity of matter is the measure of the same , arising from its
density arid hulk conjunctly.

Thus air of a double density, in a double space, is quadruple in quan-
ti ty; in a triple space, sextuple in quantity. The same thing is to be un¬
derstood of snow, and fine dust or powders, that are condensed by compres¬
sion or liquefaction ; and of all bodies that are by any causes whatever
differently condensed. I -have no regard in this place to a medium, if any
such there is, that freely pervades the interstices between the parts of
bodies. It is this quantity that I mean hereafter everywhere under the
name of body or mass. And the same is known by the weight of each *
body; for it is proportional to the weight, as I have found by experiments
on pendulums, very accurately made, which shall be shewn hereafter.

DEFINITION II.

The quantity of motion is the measure of the same . arising from the
velocity and quantity of matter conjunctly.

The motion of the whole is the sum of the motions of all the parts; and
therefore in a body double in quantity, with equal velocity, the motion is
louble; with twice the velocity, it is quadruple.

DEFINITION HI.

The vis insita, or innate force of matter , is a power of resisting , by
which every body , as much as in it lies , endeavours to persevere in its
present state , whether it be of rest , or of moving uniformly forward
in a right line.

This force is ever proportional to the body whose force it is; and differs
nothing from the inactivity of the mass, but in our manner of conceiving

THE MATHEMATICAL PRINCIPLES

T4

it. A body, from the inactivity of matter, is not without difficulty put out
of its state of rest or motion. Upon which account, this vis insita , may,
by a most significant name, be called vis inertice , or force of inactivity.
But a body exerts this force only, when another force, impressed upon it,
endeavours to change its condition; and the exercise of this force may be
considered both as resistance and impulse; it is resistance, in so far as the
body, for maintaining its present state, withstands the force impressed; it
is impulse, in so far as the body, by not easily giving way to the impressed
force of another, endeavours to change the state of that other. Resistance
is usually ascribed to bodies at rest, and impulse to those in motion;
but motion and rest, as commonly conceived, are only relatively distin¬
guished ; nor are those bodies always truly at rest, which commonly are
taken to be so.

DEFINITION IV.

An impressed force is an action exerted upon a body , in order to change
its state , either of rest , or of moving uniformly forward in a right
line.

This force consists in the action only; and remains no longer in the
Body, when the action is over. For a body maintains every new state it
acquires, by its vis inertice only. Impressed forces are of different origins •
as from percussion, from pressure, from centripetal force.

DEFINITION Y.

A centripetal force is that by irhich bodies are drawn or impelled, or any
way tend , towards a point as to a centre.

Of this sort is gravity, by which bodies tend to the centre of the earth
magnetism, by which iron tends to the loadstone; and that force, what
ever it is, by which the planets are perpetually drawn aside from the rec¬
tilinear motions, which otherwise they would pursue, and made to revolve
in curvilinear orbits. A stone, whirled about in a sling, endeavours to re¬
cede from the hand that turns it; and by that endeavour, distends the
sling, and that with so much the greater force, as it is revolved with the
greater velocity, and as soon as ever it is let go, flies away. That force
which opposes itself to this endeavour, and by which the sling perpetually
draws back the stone towards the hand, and retains it in its orbit, because
it is directed to the hand as the centre of the orbit, I call the centripetal
force. And the same thing is to be understood of all bodies, revolved in
any orbits. They all endeavour to recede from the centres of their orbits;
and wore it not for the opposition of a contrary force which restrains them
to, and detains them in their orbits, winch I therefore call centripetal, would
fly off in right lines, with an uniform motion. A projectile, if it was not
for the force of gravity, would not deviate towards the earth, tut would

OF NATUIIAL PHILOSOPHY.

75

go off from it in a right line, and that with an uniform motion,,if the re¬
sistance of the air was taken away. It is by its gravity that it is drawn
aside perpetually from its rectilinear course, and made to deviate towards
the earth, more or less, according to the force of its gravity, and the velo¬
city of its motion. The less its gravity is, for the quantity of its matter,
or the greater the velocity with which it is projected, the less will it devi¬
ate from a rectilinear course, and the farther it will go. ( If a leaden ball,
projected from the top of a mountain by the force of gunpowder with a
given velocity, and in a direction parallel to the horizon, is carried in a
curve line to the distance of two miles before it falls to the ground; the
same, if the resistance of the air were taken away, with a double or decuple
velocity, would fly twice or ten times as far. And by increasing the velo¬
city, we may at pleasure increase the distance to which it might be pro¬
jected, and diminish the curvature of the line, which it might describe, till
at last it should fall at the distance of 10, 30, or 90 degrees, or even might
go quite round the whole earth before it falls; or lastly, so that it might
never fall to the earth, but go forward into the celestial spaces, and pro¬
ceed in its motion in infinitum. And after the same manner that a pro¬
jectile, by the force of gravity, may be made to revolve in an orbit, and go
round the whole earth, the moon also, either by the force of gravity, if it
is endued with gravity, or by any other force, that impels it towards the
earth, may be perpetually drawn aside towards the earth, out of the recti¬
linear way, which by its innate force it would pursue; and would be made
to revolve in the orbit which it now describes; nor could the moon with¬
out some such force, be retained in its orbit. If this force was too small,
it would not sufficiently turn the moon out of a rectilinear course: if it
was too great, it would turn it too much, and draw down the moon from
its orbit towards the earth. It is necessary, that the force be of a just
quantity, and it belongs to the mathematicians to find the force, that may
serve exactly to retain a body in a given orbit, with a given velocity; and
vice versa , to determine the curvilinear way, into which a body projected
from a given place, with a given velocity, may be made to deviate from
its natural rectilinear way, by means of a given force.

The quantity of any centripetal force may be considered as of three
kinds; abjoluT, accelerative, and motive.

DEFINITION VI.

The absolute quantity of a centripetal force is the measure of the same

proportional to the eficacy of the cause that pi opagates it from the cen¬
tre, through the spaces round about.

Thus the magnetic force is greater in one load-stone and less in another
according to their sizes and strength of intensity.

76

THE MATHEMATICAL PRINCIPLES

DEFINITION VII.

TIw accelerative quantity of a centripetal force is the measure of tht
sa?ne, proportional to the velocity which it generates in a given time .

Thus the force of the same load-stone is greater at a less distance, and
less at a greater: also the force of gravity is greater in valleys, less on
tops of exceeding high mountains; and yet less (as shall hereafter he shown),
at greater distances from the body of the earth; but at equal distan¬
ces, it is the same everywhere; because (taking away, or allowing for, the
resistance of the air), it equally accelerates all falling bodies, whether heavy
or light, great or small.

DEFINITION VIII.

Tlie motive quantity of a centripetal force , is the measure of the sanu\

proportional to the motion which it generates in a given tinw.

Thus the weight is greater in a greater body, less in a less body; and.
in the same body, it is greater near to the earth, and less at remoter dis¬
tances. This sort of quantity is the centripetency, or propension of the
whole body towards the centre, or, as I may say, its weight; and it is al¬
ways known by the quantity of an equal and contrary force just sufficient
to Ifinder the descent of the body.

These quantities of forces, we may, for brevity's sake, call by the names
of motive, accelerative, and absolute forces; and, for distinction’s sake, con¬
sider them, with respect to the bodies that tend to the centre; to the places
of those bodies; and to the centre of force towards which they tend ; that
is to say, I refer the motive force to the body as an endeavour and propen¬
sity of the whole towards a centre, arising from the propensities of the
several parts taken together; the accelerative force to the place of the
body, as a certain power or energy diffused from the centre to all places
around to move the bodies that are in them; and the absolute force to
the centre, as endued with some cause, without which those motive forces
would not be propagated through the spaces round about; whether that
cause be some central body (su;h as is the load-stone, in the centre of the
magnetic force, or the earth in the centre of the gravitating force), or
anything else that does not yet appear. For I here design only to give a
mathematical notion of those forces, without considering their physical
causes and seats.

Wherefore the accelerative force will stand in the same relation to the
motive, as celerity does to motion. For the quantity of motion arises from
the celerity drawn into the quantity of matter; and the motive force arises
from the accelerative force drawn into the same quantity of matter. For
the sum of the actions of the accelerative force, upon the several ■ articles
of the body, is the motive force of the whole. Hence it is, that near the

OF NATURAL PHILOSOPHY.

1 ,

surface of the earth, where the accelerative gravity, or force productive of
gravity, in all bodies is the same, the motive gravity or the weight is as
the body: but if we should ascend to higher regions, where the accelerative
gravity is less, the weight would be equally diminished, and would always
be as the product of the body, by the accelerative gravity. So in those re¬
gions, where the accelerative gravity is diminished into one half, the weight
of a body two or three times less, will be four or six times less.

I likewise call attractions and impulses, in the same sense, accelerative,
and motive; and use the words attraction, impulse or propensity of any
sort towards a centre, promiscuously, and indifferently, one for another;
considering those forces not physically, but mathematically: wherefore, the
reader is not to imagine, that by those words, I anywhere take upon me to
define the kind, or the manner of any action, the causes or the physical
reason thereof, or that I attribute forces, in a true and physical sense, to
certain centres (which are only mathematical points); when at any time I
happen to speak of centres as attracting, or as endued with attractive
powers.

SCHOLIUM.

Hitherto I have laid down the definitions of such words as are less
known, and explained the sense in which I would have them to be under¬
stood in the following discourse. I do not define time, space, place and
motion, as being well known to all. Only I must observe, that the vulgar
conceive those quantities under no other notions but from the relation they
bear to sensible objects. And thence arise certain prejudices, for the re¬
moving of which, it will be convenient to distinguish them into absolute
and relative, true and apparent, mathematical and common.

I. Absolute, true, and mathematical time, of itself, and from its own na¬
ture flows equably without regard to anything external, and by another
name is called duration: relative, apparent, and common time, is some sen¬
sible and external (whether accurate or unequable) measure of duration by
the means of motion, which is commonly used instead of true time; such
as an hour, a day, a month, a year.

II. Absolute space, in its own nature, without regard to anything exter¬
nal, remains always similar and immovable. Relative space is some mo¬
vable dimension or measure of the absolute spaces; which our senses de¬
termine by its position to bodies; and which is vulgarly taken for immo¬
vable space; such is the-dimension of a subterraneous, an aereal, or celestial
space, determined by its position in respect of the earth. Absolute and
relative space, are the same in figure and magnitude; but they do not re¬
main always numerically the same. For if the earth, for instance, moves,
a space of our air, which relatively and in respect of the earth remains al¬
ways the same, will at one time be one part of the absolute space into which

rs

THE MATHEMATICAL PRINCIPLES

the air passes; at another time it will be another part of the same, and so,
absolutely understood, it will be perpetually mutable.

III. Place is a part of space which a body takes up, and is according to
the space, either absolute or relative. I say, a part of space; not the situation,
nor the external surface of the body. For the places of equal solids are
always equal; but their superfices, by reason of their dissimilar figures, are
often unequal. Positions properly have no quantity, nor are they so much
the places themselves, as the properties of places. The motion of the whole
is the same thing with the sum of the motions of the parts; that is, the
translation of the whole, out of its place, is the same thing with the sum
of the translations of the parts out of their places; and therefore the place
of the whole is the same thing with the sum of the places of the parts, and
for that reason, it is internal, and in the whole body.

IY. Absolute motion is the translation of a body from one absolute
place into another; and relative motion, the translation from one relative
place into another. Thus in a ship under sail, the relative place of a body
is that part of the ship which the body possesses; or that part of its cavity
which the body fills, and which therefore moves together with the ship :
and relative rest is the continuance of the body in the same part of the
ship, or of its cavity. But real, absolute rest, is the continuance of the
body in the same part of that immovable space, in which the ship itself,
its cavity, and all that it contains, is moved. Wherefore, if the earth is
really at rest, the body, which relatively rests in the ship, will really and
absolutely move with the same velocity which the ship has on the earth.
But if the earth also moves, the true and absolute motion of the body will
arise, partly from the true motion of the earth, in immovable space; partly
from the relative motion of the ship on the earth; and if the body moves
also relatively in the ship; its true motion will arise, partly from the true
motion of the earth, in immovable space, and partly from the relative mo¬
tions as well of the ship on the earth, as of the body in the ship; and from
these relative motions will arise the relative motion of the body on the
earth. As if that part of the earth, where the ship is, was truly moved
toward the east, with a velocity of 10010 parts; while the ship itself, with
a fresh gale, and full sails, is carried towards the west, with a velocity ex¬
pressed by 10 of those parts ; but a sailor walks in the ship towards the
east, with 1 part of the said velocity; then the sailor will be moved truly
in immovable space towards the east, with a velocity of 10001 parts, and
relatively on the earth towards the west, with a velocity of 9 of those parts.

Absolute time, in astronomy, is distinguished from relative, by the equa¬
tion or correction of the vulgar time. For the natural days are truly un¬
equal, though they are commonly considered as equal, and used for a meas¬
ure of time; astronomers correct thi3 inequality for their more accurate
deducing of the celestial motions. It may be, that there is no such thing
as an equable motion, whereby time may bo accurately measured. All mo

OF NATURAL PHILOSOPHY.

79

tions may be accelerated and retarded,; but the true, or equable, progress of
absolute time is liable to no change. The duration or perseverance of the
existence of tilings remains the same, whether the motions are swift or slow,
or none at all: and therefore it ought to be distinguished from what are
only sensible measures thereof; and out of which we collect it, by means
of the astronomical equation. The necessity of which equation, for deter¬
mining the times of a phenomenon, is evinced as well from the experiments
of the pendulum clock, as by eclipses of the satellites of Jupiter.

As the order of the parts of time is immutable, so also is the order of
the parts of space. Suppose those parts to be moved out of their places, and
they will be moved (if the expression may be allowed) out of themselves.
For times and spaces are, as it were, the places as well of themselves as of
all other things. All things are placed in time as to order of succession;
and in space as „to order of situation. It is from their essence or nature
that they are places; and that the primary places of things should be
moveable, is absurd. These are therefore the absolute places; and trans¬
lations out of those places, are the only absolute motions.

But because the parts of space cannot be seen, or distinguished from one
another by our senses, therefore in their stead we use sensible measures of
them. For from the positions and distances of things from any body con¬
sidered as immovable, we define all places; and then with respect to such
places, we estimate all motions, considering bodies as transferred from some
of those places into others. And'so, instead of absolute places and motions,
we use relative ones; and that without any inconvenience in common af¬
fairs ; but in philosophical disquisitions, we ought to abstract from our
senses, and consider things themselves, distinct from what are only sensible
measures of them. For it may be that there is no body really at rest, to
which the places and motions of others may be referred.

But we may distinguish rest and motion, absolute and relative, one from
the other by their properties, causes and effects. It is a property of rest, -
that bodies really at rest do rest in respect to one another. And therefore
as it is possible, that in the remote regions of the fixed stars, or perhaps
far beyond them, there may be some body absolutely at rest; but impossi¬
ble to know, from the position of bodies to one another in our regions
whether any of these do keep the same position to that remote body; it
follows that absolute rest cannot be determined from the position of bodies
in our regions.

It is a property of motion, that the parts, which retain given positions
to their wholes, do partake of the motions of those wholes. For all the
parts of revolving bodies endeavour to recede from the axis of motion;
and the impetus of bodies moving forward, arises from the joint impetus
of all the parts. Therefore, if surrounding bodies are moved, those that
are relatively at rest within them, will partake of their motion. Upon
which account, the true and absolute motion of a body cannot be deter-

8C

THE MATHEMATICAL PRINCIPLES

mined by the translation of it from those which only seem to rest; for the
external bodies ought not only to appear at rest, but to be really at rest,
For otherwise, all included bodies, beside their translation from near the
surrounding ones, partake likewise of their true motions; and though that
translation were not made they would not be really at rest, but only seem
to be so. For the surrounding bodies stand in the like relation to the
surrounded as the exterior part of a whole docs to the interior, or as the
shell does to the kernel; but, if the shell moves, the kernel will also
move, as being part of the whole, without any removal from near the shell.

A property, near akin to the preceding, is this, that if a place is moved,
whatever is placed therein moves along with it; and therefore a body,
which is moved from a place in motion, partakes also of the motion of its
place. Upon which account, all motions, from places in motion, are no
other than parts of entire and absolute motions; and every entire motion
is composed of the motion of the body out of its first place, and the
motion of this place out of its place; and so on, until we come to some
immovable place, as in the before-mentioned example of the sailor. Where¬
fore, entire and absolute motions can be no otherwise determined than by
immovable places: and for that reason I did before refer thoso absolute
motions to immovable places, but relative ones to movable places. Now
no other places are immovable but those that, from infinity to infinity, do „
all retain the same given position one to another; and upon this account
must ever remain unmoved; and do thereby constitute immovable space.

The causes by which true and relative motions are distinguished, one
from the other, arc the forces impressed upon bodies to generate motion.
True motion is neither generated nor altered, but by some force impressed
upon the body moved; but relative motion may be generated or altered
without any force impressed upon the body. For it is sufficient only to
impress some force on other bodies with which the former is compared,
that by their giving way, that relation may be changed, in which the re¬
lative rest or motion of this other body did consist. Again, true motion
suffers always some change from any force impressed upon the moving
body ; but relative motion does not necessarily undergo any change by such
forces. For if the same forces are likewise impressed on those other bodies,
with which the comparison is made, that the relative position may be pre¬
served, then that condition will be preserved in which the relative motion
consists. And therefore any relative motion may be changed when the
true motion remains unaltered, and the relative may be preserved when the
true suffers some change. Upon which accounts, true motion does by no
means consist in such relations.

The effects which distinguish absolute from relative motion are, the
forces of receding from the axis of circular motion. For there are no such
forces in a circular motion purely relative, but in a true and absolute cir¬
cular motion, they are greater or less, according t» the quantity of the

OF NATURAL PHILOSOPHY.

R1

motion. If a vessel, lmng by a long cord, is so often turned about that the
cord is strongly twisted, then filled Avith Avater, and held at rest together
with the Avater; after, by the sudden action of another force, it is whirled
about the contrary way, and Avhile the cord is untAvisting itself, the vessel
continues for some time in this motion; the surface of the Avater will at
first be plain, as before the vessel began to move ; but the vessel, by grad¬
ually communicating its motion to the water, Avill make it begin sensibly
t to revolve, and recede by little and little from the middle, and ascend to the
sides of the vessel, forming itself into a concave figure (as I have experi¬
enced), and the SAvifter the motion becomes, the higher Avill the Avater rise,
till at last, performing its revolutions in the same times Avith the vessel,
it becomes relatively at rest in it. This ascent of the Avater shows its en¬
deavour to recede from the axis of its motion; and the true and absolute
circular motion of the Avater, Avhich is here directly contrary to the rela¬
tive, discovers itself, and may be measured by this endeavour. At first.
Avhen the relative motion of the Avater in the vessel Avas greatest, it pro¬
duced no endeavour to recede from the axis; the Avater showed no tendency
to the circumference, nor any ascent towards the sides of the vessel, but
remained of a plain surface, and therefore its true circular motion had not
yet begun. But afterwards, Avhen the relative motion of the Avater had
decreased, the ascent thereof toAvards the sides of the vessel proved its en¬
deavour to recede from the axis; and this endeavour shoAved the real cir¬
cular motion of the Avater perpetually increasing, till it had acquired its
greatest quantity, Avhen the Avater rested relatively in the vessel. And
therefore this endeavour does not depend upon any translation of the water
in respect of the ambient bodies, nor can true circular motion be defined
by such translation. There is only one real circular motion of any one
revolving body, corresponding to only one poAver of endeavouring to recede
from its axis of motion, as its proper and adequate effect; but relative
motions, in one and the same body, are innumerable, according to the various
relations it bears to external bodies, and like other relations, are altogether
destitute of any real effect, any otherwise than they may perhaps par¬
take of that one only true motion. And therefore in their system Avho
suppose that our heavens, revolving beloAV the sphere of the fixed stars,
carry the planets along Avith them ; the several parts of those heavens, and
the planets, Avhich are indeed relatively at rest in their heavens, do yet.
really move. For they change their position one to another (Avhich never
happens to bodies truly at rest), arid being carried together Avitli their
heavens, partake of their motions, and as parts of revolving Avholes,
endeavour to recede from the axis of their motions.

Wherefore relative quantities are not the quantities themselves, Avhose
names they bear, but those sensible measures of them (cither accurate or
inaccurate), Avhich arc commonly used instead of the measured quantities
themselves. And if the meaning of Avords is to he determined by their

82

THE MATHEMATICAL PRINCIPLES

use, then by the names time, space, place and motion, their measures arv'
properly to be understood; and the expression will be unusual, and purely
mathematical, if the measured quantities themselves are meant. Upon
which account, they do strain the sacred writings, who there interpret
those words for the measured quantities. Nor do those less defile the
purity of mathematical and philosophical truths, who confound real quan¬
tities themselves with their relations and vulgar measures.

It is indeed a matter of great difficulty to discover, and effectually to
distinguish, the true motions of particular bodies from the apparent; be¬
cause the parts of that immovable space, in which those motions are per¬
formed, do by no means come under the observation of our senses. Yet
the thing is not altogether desperate: for we have some arguments to
guide us, partly from the apparent motions, which are the differences of
the true motions; partly from the forces, which are the causes and effects
of the true motions. For instance, if two globes, kept at a given distance
one from the other by means of a cord that connects them, were revolved
about their common centre of gravity, we might, from the tension of the
cord, discover the endeavour of the globes to recede from the axis of their
motion, and from thence we might compute the quantity of their circular
motions. And then if any equal forces should be impressed at once on the
alternate faces of the globes to augment or diminish their circular motions,
from the increase or deer* ase of the tensicn of I le cord, we might infer
the increment or decrement of their motions; and thence would be found
on what faces those forces ought to be impressed, that the motions of the
globes might be most augmented ; that is, we might discover their hinder-
most faces, or those which, in the circular motion, do follow. But the
faces which follow being known, and consequently the opposite ones that
precede, we should likewise know the determination of their motions. And
thus we might find both the quantity and the determination of this circu¬
lar motion, even in an immense vacuum, where there was nothing external
or sensible with which the globes could be compared. But now, if in that
space some remote bodies were placed that kept always a given position
one to another, as the fixed stars do in our regions, we could not indeed
determine from the relative translation of the globes among those bodies,
whether the motion did belong to the globes or to the bodies. But if we
observed the cord, and found that its tension was that very tension which
the motions of the globes required, we might conclude the motion to be in
the globes, and the bodies to be at rest; and then, lastly, from the trans¬
lation of the globes among the bodies, we should find the determination of
their motions. But how we are to collect the true motions from their
causes, effects, and apparent differences; and, vice versa, how from the mo¬
tions, either true or apparent, we may come to the knowledge of their
causes and effects, shall be explained more at large in the following tract
For to this end it was that I composed it.

OF NATURAL PHILOSOPHY.

83

AXIOMS, OR LAWS OF MOTION.

LAW I.

Every body perseveres in its state of rest , or of uniform motion in a right
line , unless it is compelled to change that state by forces impressed
thereon.

Projectiles persevere in their motions, so far as they are not retarded
by the resistance of the air, or impelled downwards by the force of gravity
A top, whose parts by their cohesion are perpetually drawn aside from
rectilinear motions, does not cease its rotation, otherwise than as it is re¬
tarded by the air. The greater bodies of the planets and comets, meeting
with less resistance in more free spaces, preserve theij motions both pro¬
gressive and circular for a much longer time.

LAW II.

The alteration of motion is ever proportional to the motive force impress¬
ed ; and is made in the direction of the right line in, 'which that force
is impressed.

If any force generates a motion, a double force will generate double the
motion, a triple force triple the motion, whether that force be impressed
altogether and at once, or gradually and successively. And this motion
(being always directed the same way with the generating force), if the bod y
moved before, is added to or subducted from the former motion, according
as they directly conspire with or are directly contrary to each other; or
obliquely joined, when they arc oblique, so as to produce a new motion
compounded from the determination of both.

LAW III.

To every action there is edways opposed an, equal reaction : or the mu¬
tual actions of two bodies upon each other are edways equal , and di¬
rected to contrary parts.

Whatever draws or presses another is as much drawn or pressed by that
other. If you press a stone with your finger, the finger is also pressed by
the stone. If a horse draws a stone tied to a rope, the horse (if I may so
say) will be equally drawn back towards the stone: for the distended rope,
by the same endeavour to relax or unbend itself, will draw the horse as
much towards the stone, as it does the stone towards the horse, and will
obstruct the progress of the one as much as it advances that of the other.

84

THE MATHEMATICAL PRINCIPLES

If a body impinge upon ar_other, and by its force change the motion cf lit-?
other, that body also (because of the equality of the mutual pressure) will
undergo an equal change, in its own motion, towards the contrary part.
The changes made by these actions are equal, not in the velocities but in
the motions of bodies; that is to say, if the bodies are not hindered by any
other impediments. For, because the motions are equally changed, the
changes of the velocities made towards contrary parts are reciprocally pro¬
portional to the bodies. This laiv takes place also in attractions, as will
be proved in the next scholium.

COROLLARY I.

A body by two forces conjoined will describe the diagonal of a parallelo¬
gram, in the same time that it woidd describe the sides, by those forces
apart .

If a body in a given time, by the force M impressed s -

apart in the place A, should with an uniform motion / \ /

be carried from A to B ; and by the force N impressed /

apart in the same place, should be carried from A to c i>

C; complete the parallelogram ABCD, and, by both forces acting together,
it will in the same time be carried in the diagonal from A to D. For
since the force N acts in the direction of the line AC, parallel to BD, this
force (by the second law) will not at all alter the velocity generated by the
other force M, by which the body is carried towards the line BD. The
body therefore will arrive at the line BD in the same time, whether the
rorce N be impressed or not; and therefore at the end of that time it will
be found somewhere in the line BD. By the same argument, at the end
of the same time it will be found somewhere in the line CD. Therefore it
will be found in the point D, where both lines meet. But it will move in
a right line from A to D, by Law I.

COROLLARY II.

And hence is explained the composition of any one direct force AD, out
of any two oblique forces AC and CD ; and, on tlw contrary, the re¬
solution of any one direct force AD into two oblique forces AC and
CD: which composition and resolution are abundantly confirmed from,
mechanics .

As if the unequal radii OM and ON drawn from the centre O of any
wheel, should sustain the weights A and P by the cords MA and NP; and
the forces of those weights to move the wheel were required. Through the
centre O draw the right line KOL, meeting the cords perpendicularly in
a and L; and from the centre O, with OL the greater of the distances

OF NATURAL PHILOSOPHY.

S5

OK and OL, describe a circle, meeting the cord
MA in D : and drawing CD, make AC paral- ^
lei and DC perpendicular thereto. Now, it
being indifferent whether the points K, L, D, of ' K '“
the cords be fixed to the plane of the wheel or D ^
not, the ay eights will have the same effect \
whether they are suspended from the points K
and L, or from D and L. Let the whole force
of the ay eight A be represented by the line AD,
and let it be resolved into the forces AC and W
CD ; of AYliich the force AC, draAYing the radius A
OD directly from the centre, Avill have no effect to move the AYheel: bat
the other force DC, draAving the radius DO perpendicularly, Avill have the
same effect as if it dmv perpendicularly the radius OL equal to OD ; that
is, it will have the same effect as the weight P, if that weight is to the
weight A as the force DC is to the force DA; that is (because of the sim¬
ilar triangles ADC, DOK), as OK to OD or OL. Therefore the weights A
and P, Avhich are reciprocally as the radii OK and OL that lie in the same
right line, Avill be equipollent, and so remain in equilibrio ; AYhich is the ay ell
knoAYn property of the balance, the lever, and the wheel. If either Aveight is
greater than in this ratio, its force to move the Avheel will be so much greater.

If the AYeight p, equal to the AYeight P, is partly suspended by the
cord N p, partly sustained by the oblique plane pG ; draAY pH, NH, the
former perpendicular to the horizon, the latter to the plane pG ; and if
the force of the AYeight p tending doAYnwards is represented by the line
pH, it may be resolved into the forces y?N, HN. If there AYas any plane
/?Q, perpendicular to the cord y>N, cutting the other plane pG in a line
parallel to the horizon, and the AYeight p AYas supported only by those
planes y?Q,, pG, it AYOuld press those planes perpendicularly AYith the forces
pN, HN; to AA'it, the plane pQ, AYith the force y?N, and the plane pG AYith
the force HN. And therefore if the plane AYas taken aAYay, so that
the AYeight might stretch the cord, because the cord, noAY sustaining the
AYeight, supplies the place of the plane that AYas removed, it Avill be strained
by the same force y?N AYhich pressed upon the plane before. Therefore,
the tension of this oblique cord y?N AYill be to that of the other perpendic¬
ular cord PN as y?N to pH. And therefore if the AYeight p is to the
weight A in a ratio compounded of the reciprocal ratio of the least distances
of the cords PN, AM, from the centre of the wheel, and of the direct ratio of
pH tojt?N, the weights AYill have the same effect towards moving the AYheel,
and Avill therefore sustain each other; as any one may find by experiment.

But the Aveight p pressing upon those tAYO oblique planes, may be con¬
sidered as a AYedge betAYeen the two internal surfaces of a body split by it;
and hence the ft roe* of the v.edge and the mallet may be determined; for

SG

THE MATHEMATICAL PRINCIPLES

because the force with which the weight p presses the plane pd is to the
force with which the same, whether by its own gravity, or by the blow of
a mallet, is impelled in the direction of the line jdH towards both the
planes, as joN to joH; and to the force with which it presses the other
plane pG, as joN to NH. And thus the force of the screw may be deduced
from a like resolution of forces; it being no other than a wedge impelled
with the force of a lever. Therefore the use of this Corollary spreads far
and wide, and by that diffusive extent the truth thereof is farther con¬
firmed. For on what has been said depends the whole doctrine of mechan¬
ics variously demonstrated by different authors. For from hence are easily
deduced the forces of machines, which are compounded of wheels, pullics,
fevers, cords, and weights, ascending directly or obliquely, and other mechan¬
ical powers; as also the force of the tendons to move the bones of animals.

COROLLARY III.

The quantity of motion , which is collected by taking the sum of the mo¬
tions directed towards the same parts , and the difference of those that
are directed to contrary parts , suffers no change from the action oj
bodies among themselves.

For action and its opposite re-action are equal, by Law III, and there¬
fore, by Law II, they produce in the motions equal changes towards oppo¬
site parts. Therefore if the motions are directed towards the same parts,
whatever is added to the motion of the preceding body will be subducted
from the motion of that which follows; so that the sum will be the same
as before. If the bodies meet, with contrary motions, there will be an
equal deduction from the motions of both; and therefore the difference of
the motions directed towards opposite parts will remain the same.

Thus if a spherical body A with two parts of velocity is triple of a
spherical body B which follows in the same right line with ten parts of
velocity, the motion of A will be to that of B as 6 to 10. Suppose,
then, their motions to be of 6 parts arid of 10 parts, and the sum will be
16 parts. Therefore, upon the meeting of the bodies, if A acquire 3, 4,
or 5 parts of motion, B will lose as many; and therefore after reflexion
A will proceed \Vith 9, 10, or 11 parts, and B with 7, 6, or 5 parts; the
sum remaining always of 16 parts as before. If the body A acquire 9,
10, 11, or 12 parts of motion, and therefore after meeting proceed with
15, 16, 17, or 18 parts, the body B, losing so many parts as A has got,
will either proceed with 1 part, having lost 9, or stop and remain at rest,
as having lost its whole progressive motion of 10 parts: or it will go back
with 1 part, having not only lost its whole motion, but (if 1 may so say)
one part more; or it will go back with 2 parts, because a progressive mo¬
tion of 12 parts is taken off. And so the sums of the inspiring motions
15 rl, or 164*0, and the differences of the contrary ] otions 17—1 and

OF NATURAL PHILOSOPHY.

87

IS—2, will always be equal to 16 parts, as they were before the meeting
and reflexion of the bodies. But, the motions being known with which
the bodies proceed after reflexion, the velocity of either will be also known,
by taking the velocity after to the velocity before reflexion, as the motion
after is to the motion before. As in the last case, where the motion of the
body A was of 6 parts before reflexion and of IS parts after, and the
velocity was of 2 parts before reflexion, the velocity thereof after reflexion
will be found to be of 6 parts; by saying, as the 6 parts of motion before
to IS parts after, so are 2 parts of velocity before reflexion to 6 parts after.

But if the bodies are either not spherical, or, moving in different right
lines, impinge obliquely one upon the other, and their mot'ons after re¬
flexion are required, in those cases we are first to determine the position
of the plane that touches the concurring bodies in the point of concourse,
then the motion of each body (by Corol. II) is to be resolved into two, one
perpendicular to that plane, and the other parallel to it. This done, be¬
cause the bodies act upon each other in the direction of a line perpendicu¬
lar to this plane, the parallel motions are to be retained the same after
reflexion as before; and to the perpendicular motions we are to assign
equal changes towards the contrary parts; in such manner that the sum
of the conspiring and the difference of the contrary motions may remain
the same as before. From such kind of reflexions also sometimes arise
the circular motions of bodies about their own centres. But these are
cases which I do not consider in what follows; and it would be too tedious
to demonstrate every particular that relates to this subject.

COROLLARY IV.

The common centre of gravity of two or more bodies does not alter its
state of motion or rest by the actions of the bodies among themselves ;
and therefore the common centre of gravity of all bodies acting upon
each other (excluding outward actions and impediments ) is either at
rest , or moves uniformly in a right line.

For if two points proceed with an uniform motion in right lines, and
their distance be divided in a given ratio, the dividing point will be either
at rest, or proceed uniformly in a right line. This is demonstrated here¬
after in Lem. XXIII and its Corol., when the points are moved in the same
plane; and by a like way of arguing, it may be demonstrated when the
points are not moved in the same plane. Therefore if any number of
Kdies move uniformly in right lines, the common centre of gravity of any
two of them is either at rest, or proceeds uniformly in a right line; because
the line which connects the centres of those two bodies so moving is divided at
that common centre in a given ratio. In like manner the common centre
of those two and that of a third body will be either at rest or moving uni¬
formly in a right line because at that centre the distance letween the

ss

THE MATHEMATICAL PRINCIPLES

common centre of the two bodies, and the centre of this last, is divided in
a given ratio. In like manner the common centre of these three, and of a
fourth body, is either at rest, or moves uniformly in a right line; because
the distance between the common centre of the three bodies, and the centre
of the fourth is there also divided in a given ratio, and so on in infinitum.
Therefore, in a system of bodies where there is neither any mutual action
among themselves, nor any foreign frrce impressed upon them from without,
and which consequently move uniformly in right lines, the common centre of
gravity of them all is either at rest or moves uniformly forward in a right line.

Moreover, in a system of two bodies mutually acting upon each other,
since the distances between their centres and the common centre of gravity
of both are reciprocally as the bodies, the relative motions of those bodies,
whether of approaching to or of receding from that centre, will be equal
among themselves. Therefore since the changes which happen to motions
are equal and directed to contrary parts, the common centre of those bodies,
by their mutual action between themselves, is neither promoted nor re¬
tarded, nor suffers any change as to its state of motion or rest. But in a
system of several bodies, because the common centre of gravity of any two
noting mutually upon each other suffers no change in its state by that ac¬
tion : and much less the common centre of gravity of the others with which
that action does not intervene; but the distance between those two centres
is divided by the common centre of gravity of all the bodies into parts re¬
ciprocally proportional to the total sums of those bodies whose centres they
are: and therefore while those two centres retain their state of motion or
rest, the common centre of all does also retain its state: it is manifest that
the common centre of all never suffers any change in the state of its mo¬
tion or rest from the actions of any two bodies between themselves. But
in such a system all the actions of the bodies among themselves either hap¬
pen between two bodies, or are composed of actions interchanged between
some two bodies; and therefore they do never produce any alteration in
the coinnv n centre of all as to its state of motion or rest. Wherefore
.iince that centre, when the bodies do not act mutually one upon another,
either is nt rest or moves uniformly forward in some right line, it will,
vo Withstanding the mutual actions of the bodies among themselves, always
pvYsevere in its state, either of rest, or of proceeding uniformly in a right
lino, unless it is forced out of this state by the action of some power im-
prevwd from without upon the whole system. And therefore the same law
takev place in a system consisting of many bodies as in one single body,
with ^gard to their persevering in their state of motion or of rest. For
the progressive motion, whether of one single body, or of a whole system of
bodies v*.5 always to be estimated from the motion of the centre of gravity.

COROLLARY Y.

The motions of bodies included in a given space a m e the same among

OF NATURAL PHILOSOPHY.

89

themselves, whether that space is at rest , or moves uniformly forwards
in a right line without any circular motion.

For the differences of the motions tending towards the same parts, and
the sums of those that tend towards contrary parts, are, at first (by sup¬
position), in both cases the same; and it is from those sums and differences
that the collisions and impulses do arise with which the bodies mutually
impinge one upon another. Wherefore (by Law II), the effects of those
collisions will be equal in both cases; and therefore the mutual motions
of the bodies among themselves in the one case will remain equal to the
mutual motions of the bodies among themselves in the other. A clear
proof of which we have from the experiment of a ship; where all motions
happen after the same manner, whether the ship is at rest, or is carried
uniformly forwards in a right line.

COROLLARY YI.

If bodies, any how moved among themselves, are urged in the direction
of parallel lines by equal accelerative forces, they will all continue to
move among themselves, after the same manner as if they had been
'urged by no such forces .

For these forces acting equally (with respect to the quantities of the
Dodies to be moved), and in the direction of parallel lines, will (by Law II)
move all the bodies equally (as to velocity), and therefore will never pro¬
duce any change in the positions or motions of the bodies among themselves.

SCHOLIUM.

Hitherto I have laid down such principles as have been received by math¬
ematicians, and are confirmed by abundance of experiments. By the first
two Laws and the first two Corollaries, Galileo discovered that the de¬
scent of bodies observed the duplicate ratio of the time, and that the mo¬
tion of projectiles was in the curve of a parabola; experience agreeing
with both, unless so far as these motions are a little retarded by the re¬
sistance of the air. When a body is falling, the uniform force of its
gravity acting equally, impresses, in equal particles of time, equal force's
upon that body, and therefore generates equal velocities; and in the whole
time impresses a whole force, and generates a whole velocity proportional
to the time. And the spaces described in proportional times are as the
velocities and the times conjunctly; that is, in a duplicate ratio of the
times. And when a body is thrown upwards, its uniform gravity im¬
presses forces and takes oft’ velocities proportional to the times; and the
times of ascending to the greatest heights are as the velocities to be taken
off, and those heights are as the velocities and the times conjunctly, or ir.
the duplicate ratio of the velocities. And if a body be projected in any
direction, the motion arising from its projection jS compounded with the

90

THE MATHEMATICAL PRINCIPLES

motion arising from its gravity. As if the body A by its motion of pio-
jection alone could describe in a given time the right line B

AB, and with its motion of falling alone could describe in
the same time the altitude AC; complete the paralello- j '\ E

gram ABDC ; and the body by that compounded motion \

will at the end of the time be found in the place D; and \

the curve line AED, which that body describes; will be a
parabola, to which the right line AB will be a tangent in
A; and whose ordinate BD will be as the square of the line AB. On the
same Laws and Corollaries depend those things which have been demon- **
strated concerning the times of the vibration of pendulums, and are con¬
firmed by the daily experiments of pendulum clocks. By the same, to¬
gether with the third Law, Sir Christ. Wren, Dr. Wallis, and Mr. Huy¬
gens, the greatest geometers of our times, did severally determine the rules
of the congress and reflexion of hard bodies, and much about the same
time communicated their discoveries to the Royal Society, exactly agreeing
among themselves as to those rules. Dr. Wallis, indeed, was something
more early in the publication; then followed Sir Christopher Wren, and,
lastly, Mr. Huygens. But Sir Christopher Wren confirmed the truth of
the thing before the Royal Society by the experiment of pendulums, which
Mr. Mariotte soon after thought fit to explain in a treatise entirely upon
that subject. But to bring this experiment to an accurate agreement with
the theory, we are to have a due regard as well to the resistance of the air
as to the elastic force of the concurring bodies. Let the spherical bodies
A, B be suspended by the parallel and e G c D P II

equal strings AC, BD, from the centres
C, D. About these centres, with those n
intervals, describe the semicircles EAP,

GBH, bisected by the radii CA, DB.

Bring the body A to any point R of the
arc EAF, and (withdrawing the body
B) let it go from thence, and after one oscillation suppose it to return to
the point V: then RY will be the retardation arising from the resistance
of the air. ‘Of this RY let ST be a fourth part, situated in the middle,
to wit, so as RS and TY may be equal, and RS may be to ST as 3 to 2
then will ST represent very nearly the retardation during the descent
from S to A. Restore the body B to its place : and, suppesing the body
A to be let fall from the point S, the velocity thereof in the place of re¬
flexion A, without sensible error, will be the same as if it had descended
in vacuo from the point T. Upon which account this velocity may be
represented by' the chord of the arc TA. For it is a proposition well
known to geometers, that the velocity of a pendulous body in the lowest
point is as the chord of the arc which it has described in its descent. After

OF NATURAL PHILOSOPHY.

9 I

reflexion, suppose the body A comes to the place s, and the body B to the
place k. Withdraw the body B, and find the place v, from which if the
body A, being let go, should after one oscillation return to the place r, st
may be a fourth part of rv , so placed in the middle thereof as to leave rs
equal to tv, and let the chord of the arc tA represent the velocity which
the body A had in the place A immediately after reflexion. For t will be
the true and correct place to which the body A should have ascended, if
the resistance of the air had been taken off. In the s.ime way we are to
correct the place k to which the body B ascends, by finding the place l to
which it should have ascended in vacuo. And thus everything may be
subjected to experiment, in the same manner as if we were really placed
in vacuo. These things being done, we are to take the product (if I may
so say) of the body A, by the chord of the arc TA (which represents its
velocity), that we may have its motion in the place A immediately before
reflexion; and then by the chord of the arc tA, that we may have its mo¬
tion in the place A immediately after reflexion. And so we are to take
the product of the body B by the chord of the arc B l, that we may have -
the motion of the same immediately after reflexion. And in like manner,
when two bodies are let go together from different places, we are to find
the motion of each, as well before as after reflexion; and then we may
compare the motions between themselves, and collect the effects of the re¬
flexion. Thus trying the thing with pendulums of ten feet, in unequal
as well as equal bodies, and making the bodies to concur after a descent
through large spaces, as of 8, 12, or 16 feet, I found always, without an
error of 3 inches, that when the bodies concurred together directly, equal
changes towards the contrary parts were produced in their motions, and,
of consequence, that the action and reaction were always equal. As if the
body A impinged upon the body B at rest with 9 parts of motion, and
losing 7, proceeded after reflexion with 2, the body B was carried back¬
wards with those 7 parts. If the bodies concurred with contrary motions,

A with twelve parts of motion, and B with six, then if A receded with 2,

B receded with 8; to wit, with a deduction of 14 parts of motion on
, each side. For from the motion of A subducting twelve parts, nothing
will remain; but subducting 2 parts more, a motion will be generated of
2 parts towards the contrary way; and so, from the motion of the body
B of 6 parts, subducting 14 parts, a motion is generated of S parts towards
the contrary way. But if the bodies were made both to move towards the
same way, A, the swifter, with 14 parts of motion, B, the slower, with 5,
and after reflexion A went on with 5, B likewise went on with 14 parts;

9 parts being transferred from A to B. And so in other cases. By the
congress and collision of bodies, the quantity of motion, collected from the
sum of the motions directed towards the same way, or from the difference
of those that were directed towards contrary ways, was never changed.
For the error of an inch or two in measures may be easily ascribed to the

92

THE MATHEMATICAL PRINCIPLES

difficulty of executing everything with accuracy. It was not easy to let
go the two pendulums so exactly together that the bodies should impinge
one upon the other in the lowermost place AB; nor to mark the places s,
and k, to which the bodies ascended after congress. Nay, and some errors,
too, might have happened from the unequal density of the parts of the pen¬
dulous bodies themselves, and from the irregularity of the texture pro¬
ceeding from other causes.

But to prevent an objection that may perhaps be alledged against the
rule, for the proof of which this experiment was made, as if this rule did
suppose that the bodies were either absolutely hard, or at least perfectly
elastic (whereas no such bodies are to be found in nature), I must add, that
the experiments we have been describing, by no means depending upon
that quality of hardness, do succeed as well in soft as in hard bodies. For
if the rule is to be tried in bodies not perfectly hard, we are only to di¬
minish the reflexion in such a certain proportion as the quantity of the
elastic force requires. By the theory of Wren and Huygens, bodies abso¬
lutely hard return one from another with the same velocity with which
they meet. But this may be aflirmed with more certainty of bodies per¬
fectly elastic. In bodies imperfectly elastic the velocity of the return is to
be diminished together with the elastic force; because that force (except
when the parts of bodies are bruised by their congress, or suffer some such
extension as happens under the strokes of a hammer) is (as far as I can per¬
ceive) certain and determined, and makes the bodies to return one from
the other with a relative velocity, which is in a given ratio to that relative
velocity with which they met. This I tried in balls of wool, made up
tightly, and strongly compressed. For, first, by letting go the pendulous
bodies, and measuring their reflexion, I determined the quantity of their
elastic force; and then, according to this force, estimated the reflexions
that ought to happen in other cases of congress. And with this computa¬
tion other experiments made afterwards did accordingly agree; the balls
always receding one from the other with a relative velocity, which was to
the relative velocity with which they met as about 5 to 9. Balls of steel
returned with almost the same velocity : those of cork with a velocity some-^
thing less; but in balls of glass the proportion was as about 15 to 16.
And thus the third Law, so far as it regards percussions and reflexions, is
proved by a theory exactly agreeing with experience.

In attractions, I briefly demonstrate the thing after this manner. Sup¬
pose an obstacle is interposed to hinder the congress of any two bodies A,
B, mutually attracting one the other: then if either body, as A, is more
attracted towards the other body B, than that other body B is towards the
first body A, the obstacle will be more strongly urged by the pressure of
the body A than by the pressure of the body B, and therefore will not
remain in equilibrio : but the stronger pressure will prevail, and will make
the system of the two bodies, together with the obstacle, to move directly

OF NATURAL PHILOSOPHY.

93

towards the parts on which B lies: and in free spaces, to go forward in
infinitum with a motion perpetually accelerated; which is absurd and
contrary to the first Law. For, by the first Law, the system ought to per¬
severe in its state of rest, or of moving uniformly forward in a right line;
and therefore the bodies must equally press the obstacle, and be equally
attracted one by the other. I made the experiment on the loadstone and
iron. If these, placed apart in proper vessels, are made to float by one
another in standing water, neither of them will propel the other; but,
by being equally attracted, they will sustain each other's pressure, and rest
at last in an equilibrium.

So the gravitation betwixt the earth and its parts is mutual. Let the
earth FI be cut by any plane EG into two parts EOF
and EGI, and their weights one towards the other
will be mutually equal. For if by another plane
HK, parallel to the former EG, the greater part F
EGI is cut into two parts EGKH and HKI.
whereof HKI is equal to the part EFG, first cut
off, it is evident that the middle part EGKH, will
have no propension by its proper weight towards either side, but will hang
as it were, and rest in an equilibrium betwixt both. But the one extreme
part HKI will with its whole weight bear upon and press the middle part
towards the other extreme part EGF: and therefore the force with which
EGI, the sum of the parts HKI and EGKH, tends towards the third part
EGF, is equal to the weight of the part HKI, that is, to the weight of
the third part EGF. And therefore the weights of the two parts EGI
and EGF, one towards the other, are equal, as I was to prove. And in¬
deed if those weights were not equal, the whole earth floating in the non¬
resisting aether would give wmy to the greater weight, and, retiring from
it, would be carried off in infinitum.

And as those bodies are equipollent in the congress and reflexion, whose
velocities are reciprocally as their innate forces, so in the use of mechanic
instruments those agents are equipollent, and mutually sustain each the
contrary pressure of the other, whose velocities, estimated according to the
determination of the forces, are reciprocally as the forces.

So those weights are of equal force to move the arms of a balance;
which during the play of the balance are reciprocally as their velocities
up^ ards and downwards; that is, if the ascent or descent is direct, those
weights are of equal force, which are reciprocally as the distances of the
points at which they are suspended from the axis ol the balance; but if
they are turned aside by the interposition of oblique planes, or other ob¬
stacles, and made to ascend or descend obliquely, those bodies will be
equipollent, which are reciprocally as the heights of their ascent and de¬
scent taken according .to the perpendicular; and that on account of the
determination of gravity downwards.

94

THE MATHEMATICAL PRINCIPLES

.And in like manner in the pully, or in a combination of pullies, the
force of a hand drawing the rope directly, which is to the weight, whethei
ascending directly or obliquely, as the velocity of the perpendicular ascent
of the weight to the velocity of the hand that draws the rope, will sustain
the weight.

In clocks and such like instruments, made up from a combination of
wheels, the contrary forces that promote and impede the motion of the
wheels, if they are reciprocally as the velocities of the parts of the wheel
en which they are impressed, will mutually sustain the one the other.

The force of the screw to press a body is to the force of the hand that
turns the handles by which it is moved as the circular velocity of the
handle in that part where it is impelled by the hand is to the progressive
velocity of the screw towards the pressed body.

The forces by which the wedge presses or drives the two parts of the
wood it cleaves are to the force of the mallet upon the wedge as the pro¬
gress of the wedge in the direction of the force impressed upon it by the
mallet is to the velocity with which the parts of the wood yield to the
wedge, in the direction of lines perpendicular to the sides of the wedge.
And the like account is to be given of all machines.

The power and use of machines consist only in this, that by diminishing
the velocity we may augment the force, and the contrary: from whence
in all sorts of proper machines, we have the solution of this problem; 7.
move a given weight with a given power , or with a given force to over¬
come any other given resistance. For if machines are so contrived that the
velocities of the agent and resistant are reciprocally as their forces, the
agent will just sustain the resistant, hut with a greater disparity of ve¬
locity will overcome it. So that if the disparity of velocities is so great
as to overcome all that resistance which commonly arises either from the
attrition of contiguous bodies as they slide by one another, or from the
cohesion of continuous bodies that are to be separated, or from the weights
of bodies to be raised, the excess of the force remaining, after all those re¬
sistances are overcome, will produce an acceleration of motion proportional
thereto, as well in the parts of fhe machine as in the resisting body. But
to treat of mechanics is not my present business. I was only willing to
show by those examples the great extent and certainty of the third Law ot
motion. For if we estimate the action of the agent from its force and
velocity conjunctly, and likewise the reaction of the impediment conjunctly
from the velocities of its several parts, and from the forces of resistance
arising from the attrition, cohesion, weight, and acceleration of those parts,
the action and reaction in the use of all sorts of machines will found
always equal to one another. And so far as the action is propagated by
the intervening instruments, and at last impressed upon ti e resisting
body, the ultimate determination of the action will be always contrary to
the determination of the reaction.

OF NATURAL PHILOSOPHY.

95

BOOK I.

OF THE MOTION OF BODIES.

SECTION I.

Of the method of first and last ratios of quantities, by the help whereof
we demonstrate the propositions that follow.

LEMMA I.

Quantities, and the ratios of quantities, which in any finite time converge
continually to equality, and before the end of that time approach nearer
the one to the other than by any given difference, become ultimately
equal.

If you deny it, suppose them to be ultimately unequal, and let D be
their ultimate difference. Therefore they cannot approach nearer to
equality than by that given difference D ; which is against the supposition.

cr

f

in.

71

LEMMA II.

If in any figure A acE, terminated by the right
lines A a, AE, and the curve acE, there be in¬
scribed any number of parallelograms Ab, Be,

Cd, ej’c., comprehended, under equal bases AB,

BC, CD, $ *c., and the sides, Bb, Cc, Dd, 4*c.,
parallel to one side Aa of the figure; and the
parallelograms aKbl, bLcm, cMdn, §'c., are com¬
pleted. Then if the breadth of those parallelo¬
grams be supposed to be diminished, and their a BF C D E
number to be augmented in infinitum; I say, that :he ultimate ratios
which the inscribed figure AKbLcMdD, the tin umscribed figure
AalbmcndoE, and curvilinear figure AabcdE, will have to one another ,
are ratios of equality.

For the difference of the inscribed and circumscribed figures is the sum
of the parallelograms Kl, L m, M//, Do, that is (from the equality of all
their bases), the rectangle under one of their bases K6 and the sum of their

a

\—
\

A

altitudes Aa, that is, the

rectangle

ABla. But this rectangle, because

96

THE MATHEMATICAL PRINCIPLES

_ [Book 1

its breadth AB is supposed diminished - in infinitum, becomes less than
any given space. And therefore (by Lem. I) the figures inscribed and
circumscribed become ultimately equal one to the other; and much more
will the intermediate curvilinear figure be ultimately equal to either.
Q.E.D.

LEMMA III.

The same ultimate ratios arc also ratios of equality, when the breadths ,
AB, BC, DC, $'c., of the parallelograms are unequal, and are all di¬
minished in infinitum.

For suppose AF equal to the greatest breadth, and
complete the parallelogram FA af. This parallelo¬
gram will be greater than the difference of the in¬
scribed and circumscribed figures; but, because its
breadth AF is diminished in infinitum, it will be¬
come less than any given rectangle. Q.E.D.

Cor. 1. Hence the ultimate sum of those evanes¬
cent parallelograms will in all parts coincide with

the curvilinear figure. A BF C D E

Cor. 2. Much more will the rectilinear figure # comprehended under tne
chords of the evanescent arcs ab, be, cd, &c., ultimately coincide with tl.c
curvilinear figure.

Cor. 3. And also the circumscribed rectilinear figure comprehended
under the tangents of the same arcs.

Cor. 4 And therefore these ultimate figures (as to their perimeters acE)
are not rectilinear, but curvilinear limits of rectilinear figures.

LEMMA IV.

If in two figures AacE, PprT, you inscribe {as before)
two ranks of parallelograms, an equal number in
each rank, and, when their breadths are diminished
in infinitum, theultimate ratios of the parallelograms
in one figure to those in the other, each to each respec¬
tively, are the same ; I say, that those two figures
AacE, PprT, are to one another in that same ratio .

For as the parallelograms in the one are severally to
the parallelograms in the other, so (by composition) is the &
sum of all in the one to the sum of all in the other: and
so is the one figure to the other; because (by Lem. Ill) the
former figure to the former sum, and the latter figure to the
latter sum, are both in the ratio of equality. Q.E.D.

Cor. Hence if two quantities of any kind are any
how divided into an equal number of parts, and those a

OF NATURAL PHILOSOPHY.

9?

Sec. 1.1

parts, when their number is augmented, and their magnitude diminished
in infinitum, have a given ratio one to the other, the first to the first, the
second to the second, and so on in order, the whole quantities will be one to
the other in that same given ratio. For if, in the figures of this Lemma,
the parallelograms are taken one to the other in the ratio of the parts, the
sum of the parts will always be as the sum of the parallelograms; and
therefore supposing the number of the parallelograms and parts to be aug¬
mented, and their magnitudes diminished in infinitum.\, those sums will be
in the ultimate ratio of the parallelogram in the one figure to the corres¬
pondent parallelogram in the other; that is (by the supposition), in the
ultimate ratio of any part of the one quantity to the correspondent part of
the other.

LEMMA Y.

In similar figures, all sorts of homologous sides, ivhether curvilinear 07
rectilinear, are proportional; and the areas are in the duplicate ratio
of the homologous sides.

LEMMA VI.

If any arc ACB, given in position is sub¬
tended by its chord AB, and in any point
A, in the middle of the continued curva¬
ture, is touched by a right line AD, pro¬
duced both ways; then if the points A
and B approach one another and meet,

I say, the angle B AD, contained betiocen
the chord and. the tangent, will be dimin¬
ished in infinitum, and ultimately will vanish.

For if that angle does not vanish, the arc ACB will contain with the
tangent AD an angle equal to a rectilinear angle; and therefore the cur¬
vature at the point A will not be continued, which is against the supposi¬
tion.

LEMMA VII.

The same things being supposed, I say that the ultimate ratio of the arc ,
chord, and tangent, any one to any other, is the ratio of equality.

For while the point B approaches towards the point A, consider always
AB and AD as produced to the remote points b and d, and parallel to the
secant BD draw bd : and let the arc Acb be always similar to the arc
ACB. Then, supposing the points A and B to coincide, the angle dAb
will vanish, by the preceding Lemma; and therefore the right lines A b,
Ad (which are always finite), and the intermediate arc Acb, will coincide,
and become equal among themselves. Wherefore, the right lines AB, AD.

98 THE MATHEMATICAL PRINCIPLES [SeC. I.

and the intermediate arc ACB (which are always proportional to the
former), will vanish, and ultimately acquire the ratio of equality. Q.E.D.

Cor. 1. Whence if through B we draw
BP parallel to the tangent, always cutting
any right line A P passing through A in
P, this line BP will be ultimately in the
ratio of equality with the evanescent arc ACB; because, completing the
parallelogram AFBD, it is always in a ratio of equality with AD.

Cor. 2. And if through B and A more right lines are drawn, as BE,
BD, AP, AG, cutting the tangent AD and its parallel BP; the ultimate
ratio of all the abscissas AD, AE, BP, BG, and of the chord and arc AB,
any one to any other, will be the ratio of equality.

Cor. 3. And therefore in all our reasoning about ultimate ratios, we
may freely use any one of those lines for any other.

LEMMA VIII.

If the right lines AR, BR, with the arc ACB, the chord AB, and the
tangent AD, constitute three triangles RAB. RACB, RAD,' and the
points A and B approach and meet: I say, that the ultimate form of
these evanescent triangles is that of similitude , and their ultimate
ratio that of equality.

Por while the point B approaches towards
the point A, consider always AB, AD, AR,
as produced to the remote points b, d , and r,
and rbd as drawn parallel to RD, and let
the arc A cb be always similar to the arc
ACB. Then supposing the points A and B
to coincide, the angle bAd will vanish; and
therefore the three triangles rAb, rAcb, rAd
(which are always finite), will coincide, and on that account become both
similar and equal. And therefore the triangles RAB, RACB, RA D
which are always similar and proportional to these, will ultimately be¬
come both similar and equal among themselves. Q..E.D.

Cor. And hence in all reasonings about ultimate ratios, we may indif¬
ferently use any one of those triangles for any other.

LEMMA IX.

If a ngnt line AE. and a curve line ABC, both given by position , cut
each other m a given angle , A; and to that right line, in another
given angle, BD, CE are ordinately applied, meeting the curve in B,
C: and the points B and C together approach towards and meet in
the point A: I say, that the areas of the triangles ABD, ACE, wilt
ultimately be one to the other in the duplicate ratio of the sides.

a e\ in

Book I.|

OF NATURAL PHILOSOPHY.

99

For while the points B, C, approach e_
towards the point A, suppose always AD
to be produced to the remote points d and

e, so as Ad, Ae may be proportional to
AD, AE; and the ordinates db, ec, to be
drawn parallel to the ordinates DB and E
EC, and meeting AB and AC produced d[
in b and c. 1 iet the curve Abe be similar
to the curve ABC, and draw the rio;ht line
A g so as to touch both curves in A, and
cut the ordinates DB, EC, db ec, in F, G,

f, g. Then, supposing the length Ae to remain the same, let the points B
and C meet in the point A ; and the angle cAg vanishing, the curvilinear
areas Abd, Ace will coincide with the rectilinear areas A fd, Age\ and
therefore (by Lem. V) will be one to the other in the duplicate ratio of
the sides Ad, Ae. But the areas ABD, ACE are always proportional to
these areas; and so the sides AD, AE are to these sides. And therefore
the areas ABD, ACE are ultimately one to the other in the duplicate ratio
of the sides AD, AE. Q.E.D.

LEMMA X.

The spaces which a body describes by any finite force urging it, whether
that force is determined and immutable, or is continually augmented
or continually diminished, are in the very beginning of the mMion one
to the other in the duplicate ratio of the times .

Let the times be represented by the lines AD, AE, and the velocities
generated in those times by the ordinates DB, EC. The spaces described
with these velocities will be as the areas ABD, ACE, described by those
ordinates, that is, at the very beginning of the motion (by Lem. IX), in
the duplicate ratio of the times AD, AE. Q.E.D.

Cor. 1. And hence one may easily infer, that the errors of bodies des¬
cribing similar parts of similar figures in proportional times, are nearly
as the squares of the times in which they are generated; if so be these
errors are generated by any equal forces similarly applied to the bodies,
and measured by the distances of the bodies from those places of the sim¬
ilar figures, at which, without the action of those forces, the bodies would
have arrived in those proportional times.

Cor. 2. But the errors that are generated by proportional forces, sim¬
ilarly applied to the bodies at similar parts of the similar figures, are as
the forces and the squares of the times conjunc tly.

Cor. 3. The same thing is to be understood of any spaces whatsoever
described by bodies urged with different forces; all which, in the very be¬
ginning of the motion, are as the forces and the squares of the times conjunctlv.

100

THE MATHEMATICAL PRINCIPLES

[Sec. 1

Cor. 4. And therefore the forces are as the spaces described in the very
beginning of the motion directly, and the squares of the times inversely.

Cor. 5. And the squares of the times are as the spaces described direct¬
ly, and the forces inversely.

SCHOLIUM.

If in comparing indetermined quantities of different sorts one with
another, any one is said to be as any other directly or inversely, the mean¬
ing is, that the former is augmented or diminished in the same ratio with
the latter, or with its reciprocal. And if any one is said to be as any other
two or more directly or inversely, the meaning is, that the first is aug¬
mented or diminished in the ratio compounded of the ratios in which the
others, or the reciprocals of the others, are augmented or diminished. As
if A is said to be as B directly, and C directly, and D inversely, the mean¬
ing is, that A is augmented or diminished in the same ratio with B X C
X qy-, that is to say, that A and ^ are one to the other in a given ratio.

LEMMA XI.

The evanescent subtense of the angle of contact, in all curves which at
the point of contact have a finite curvature, is ultimately in the dupli¬
cate rath of the subtense of the conterminate arc.

Case 1 . Let AB be that arc, AD its tangent, BD
the subtense of the angle of contact perpendicular on
the tangent, AB the subtense of the arc. Draw BG
perpendicular to the subtense AB, and AG to the tan¬
gent AD, meeting in G; then let the points D, B, and
O, approach to the points d, b, and g, and suppose J
to be the ultimate intersection of the lines BG, AG,
when the points D, B, have come to A. It is evident
that the distance GJ may be less than any assignable.'

But (from the nature of the circles passing through g
the points A, B, G, A, b, g,) AB 2 = AG X BD, and
Ab 2 = Ag X bd; and therefore the ratio of AB 2 to A b 2 is compounded of
the ratios of AG to Ag, and of B d to bd. But because GJ may be as¬
sumed of less length than any assignable, the ratio of AG to Ag may be
such as to differ from the ratio of equality by less than any assignable
difference; and therefore the ratio of AB 2 to Ab 2 may be such as to differ
from the ratio of BD to bd by less than any assignable difference. There¬
fore, by Lem. I, the ultimate ratio of AB 2 to Ab 2 is the same with tb.o ul¬
timate ratio of BD to bd. Q.E.D.

Case 2. Now let BD be inclined to AD in any given angL, and the
ultimate ratio of BD to bd will always be the same as before, and there¬
fore the same with the ratio of AB 2 to Ab 2 . Q.E.D

OF NATURAL PHILOSOPHY.

101

Book I.] -

Case 3. And if we suppose the angle D not to he given, but that the
right line BD converges to a given point, or is determined by any other
condition whatever ; nevertheless the angles D, d, being determined by the
same law, will always draw nearer to equality, and approach nearer to
each other than by any assigned difference, and therefore, by Lem. I, will at
last be* equal; and therefore the lines BD, bd arc in the same ratio to each
other as before. Q.E.D.

Cor. 1. Therefore since the tangents AD, A d, the arcs AB, A b, and
their sines, BC, be , become ultimately equal to the chords AB, A b, their
squares will ultimately become as the subtenses BD, bd.

Cor. 2. Their squares are also ultimately as the versed sines of the arcs,
bisecting the chords, and converging to a given point. For those versed
sines are as the subtenses BD, bd.

Cor. 3. And therefore the versed sine is in the duplicate ratio of the
time in which a body will describe the arc with a given velocity.

Cor. 4. The rectilinear triangles ADB, A db are cl D

ultimately in the triplicate ratio of the sides AD, Ad,
and in a sesquiplicate ratio of the sides DB, db; as
being in the ratio compounded of the sides AD to DB,
and of Ad to db. So also the triangles ABC, A be
are ultimately in the triplicate ratio of the sides BC, be.

What I call the sesquiplicate ratio is the subduplicate
of the triplicate, as being compounded of the simple
and subduplicate ratio.

Cor. 5. And because DB, db are ultimately paral- gr
lei and in the duplicate ratio of the lines AD, Ad, the i
ultimate curvilinear areas ADB, A db will be (by the nature of the para*
bola) t\vo thirds of the rectilinear triangles ADB, A db and the segments
AB, A b will be one third of the same triangles. And thence those areas
and those segments will be in the triplicite ratio as well of the tangents
AD, Ad, as of the chords and arcs AB, AB.

SCHOLIUM.

But we have all along supposed the angle of contact to be neither infi¬
nitely greater nor infinitely less than the angles of contact made by cir¬
cles and their tangents: that is, that the curvature at the point A is neither
infinitely small nor i afinitely great, or that the interval AJ is of a finite mag¬
nitude. For DB may be taken as AD 3 : in which case no circle can be drawn
through the point A, between the tangent AD and the curve AB, and
therefore the angle of contact will be infinitely less than those of circles.
And by a like reasoning, if DB be made successfully as AD 4 , AD 5 , AD 8 ,
AD 7 , (fee., we shall have a series of angles of contact, proceeding in infini¬
tum, wherein every succeeding term is infinitely less than the pre-

102

THE MATHEMATICAL PRINCIPLES

[Book 1

ceding. And if DB be made successively as AD 2 , ADf, AD^, AD], AD|
AD], &c., we shall have another infinite series of angles of contact, the first
of which is of the same sort with those of circles, the second infinitely
greater, and every succeeding one infinitely greater than the preceding.
But between any two of these angles another series of intermediate angles
of contact may be interposed, proceeding both ways in infinitum, wherein
every succeeding angle shall be infinitely greater or infinitely less than the
preceding. As if between the terms AD 2 and AD 3 there were interposed
the series AD 13 , AD£ AD 3 , AD], AD], ADJ, AD^ 1 , AD£ ADf, &c. And
again, between any two angles of this series, a new series of intermediate
angles may be interposed, differing from one another by infinite intervals.
Nor is nature confined to any bounds.

Those things which have been demonstrated of curve lines, and the
superfices which they comprehend, may be easily applied to the curve su-
perfices and contents of solids. These Lemmas are premised to avoid the
tediousness of deducing perplexed demonstrations ad absurdum, according
to the method of the ancient geometers. For demonstrations are more
contracted by the method of indivisibles: but because the hypothesis of
indivisibles seems somewhat harsh, and therefore that method is reckoned
less geometrical, I chose rather to reduce the demonstrations of the follow¬
ing propositions to the first and last sums and ratios of nascent and evane¬
scent quantities, that is, to the limits of those sums and ratios; and so to
premise, as short as I could, the demonstrations of those limits. For hereby
the same thing is performed as by the method of indivisibles; and now
those principles being demonstrated, we may use them with more safety.
Therefore if hereafter I should happen to consider quantities as made up of
particles, or should use little curve lines for right ones, I would not be un¬
derstood to mean indivisibles, but evanescent divisible quantities : not the
sums and ratios of determinate parts, but always the limits of sums and
ratios; and that the force of such demonstrations always depends on the
method laid down in the foregoing Lemmas.

Perhaps it may be objected, that there is no ultimate proportion, of
evanescent quantities; because the proportion, before the quantities have
vanished, is not the ultimate, and when they are vanished, is none. But
by the same argument, it may be alledged, that a body arriving at a cer¬
tain place, and there stopping, has no ultimate velocity: because the velo¬
city, before the body comes to the place, is not its ultimate velocity ; when
it has arrived, is none 1 ut the answer is easy; for by the ultimate ve¬
locity is meant that with which the body is moved, neither before it arrives
at its last place and the motion ceases, nor after, but at the very instant it
arrives ; that is, that velocity with which the body arrives at its last place,
and with which the motion ceases. And in like manner, by the ultimate ra¬
tio of evanescent quantities is to le understood the ratio of the quantities

OF NATURAL PHILOSOPHY.

103

Sec. II.1

-i

Dot before they vanish, nor afterwards, but with which they vanish. In
like manner the first ratio of nascent quantities is that with which they begin
to be. And the first or last sum is that with which they begin and cease
to be (or to be augmented or diminished). There is a limit which the ve¬
locity at the end of the motion may attain, but not exceed. 'This is the
ultimate velocity. And there is the like limit in all quantities and pro¬
portions that begin and cease to be. And since such limits are certain and
definite, to determine the same is a problem strictly geometrical. But
whatever is geometrical we may be allowed to use in determining and de¬
monstrating any other thing that is likewise geometrical.

It may also be objected, that if the ultimate ratios of evanescent quan¬
tities are given, their ultimate magnitudes will be also given: and so all
quantities will consist of indivisibles, which is contrary to what Euclid
has demonstrated concerning incommensurables, in the 1.0th Book of his
Elements. But this objection is founded on a false supposition. For
those ultimate ratios with w'hich quantities vanish are not truly the ratios
of ultimate quantities, but limits towards which the ratios of quantities
decreasing without limit do always converge; and to which they approach
nearer than by any given difference, but never go beyond, nor in effect attain
to, till the quantities are diminished in infinitum. This thing will appear
more evident in quantities infinitely great. If two quantities, whose dif¬
ference is given, be augmented in infinitum , the ultimate ratio of these
quantities will be given, to wit, the ratio of equality; but it does not from
thence follow, that the ultimate or greatest quantities themselves, whose
ratio that is, will be given. Therefore if in what follows, for the sake of
being more easily understood, I should happen to mention quantities as
least, or evanescent, or ultimate, you are not to suppose that quantities of
any determinate magnitude are meant, but such as are conceived to be al¬
ways diminished without end.

SECTION II.

Of the Invention of Centripetal Forces .

PROPOSITION I. THEOREM I.

The areas, which revolving bodies describe by radii draivn to an immo¬
vable centre of force do lie in the same immovable planes, and are pro¬
portional to the times in which they are described.

For suppose the time to be divided into equal parts, and in the first part
of that time let the body by its innate force describe the right line AB
In the second part of that time, the same would (by Law I.), if not hindered,
proceel directly to c, alo iq: the line Be equal to AB ; so that by the radii
AS, BS, cS, draw.i to the centre, the equal areas ASB, BSc, would be de-

104

THE MATHEMATICAL PRINCIPLES

[Book l

scribed. But when the body
is arrived at B, suppose
that a centripetal force acts
at once with a great im¬
pulse, and, turning aside the
body from the right line Be,
compels it afterwards to con¬
tinue its motion along the
right line BC. Draw cC
parallel to BS meeting BC
in C; and at the end of the
second part of the time, the
body (by Cor. I. of the Laws)
will be found in C, in the
same plane with the triangle
A SB. Join SC, and, because
SB and C c are parallel, the triangle SBC will be equal to the triangle SBc,
and therefore also to the triangle SAB. By the like argument, if the
centripetal force acts successively in C, D, E, &c., and makes the body, in
each single particle of time, to describe the right lines CD, DE, EF, &c.,
they will all lie in the same plane; and the triangle SCD will be equal to
the triangle SBC, and SDE to SCD, and SEF to SDE. And therefore,
in equal times, equal areas are described in one immovable plane; and, by
composition, any sums SADS, SAFS, of those areas, are one to the other
as the times in which they are described. Now let the number of those
triangles be augmented, and their breadth diminished in infinitum,; and
(by Cor. 4, Lem. III.) their ultimate perimeter ADF will be a curve line:
and therefore the centripetal force, by which the body is perpetually drawn
back from the tangent of this curve, will act continually; and any described
areas SADS, SAFS, which are always proportional to the times of de¬
scription, will, in this case also, be proportional to those times. Q.E.D.

Cor. 1. The velocity of a body attracted towards an immovable centre,
in spaces void of resistance, is reciprocally as the perpendicular let fall
from that centre on the right line that touches the orbit. For the veloci¬
ties in those places A, B, C, D, E, are as the bases AB, BC, CD, DE, EF.
of equal triangles ; and these bases are reciprocally as the perpendiculars
let fall upon them.

Cor. 2. If the chords AB, BC of two arcs, successively described in
equal times by the same body, in spaces void of resistance, are completed
into a parallelogram ABCV, and the diagonal BY of this parallelogram,
in the position which it ultimately acquires when those arcs are diminished
in infinitum ,, is produced both ways, it will pass through the centre of force.

Cor. 3. If the chords AB, BC, and DE, EF, cf arcs described in equal

Sec. IT.]

OF NATURAL PHILOSOPHY.

105

times, in spaces void of resistance, are completed into the parallelograms
ABCY, DEFZ : the forces in B and E are one to the other in the ulti¬
mate ratio of the diagonals BY, EZ, when those arcs are diminished in
infinitum. For the motions BC and EF of the body (by Cor. 1 of the
Laws) are compounded of the motions Be, BY, and E f } EZ : but BY and
EZ, which are equal to Cc and Ff in the demonstration of this Proposi¬
tion, were generated by the impulses of the centripetal force in B and E,
and are therefore proportional to those impulses.

Cor. 4. The forces by which bodies, in spaces void of resistance, are
drawn back from rectilinear motions, and turned into curvilinear orbits,
are one to another as the versed sines of arcs described in equal times; which
versed sines tend to the centre of force, and bisect the chords when those
arcs are diminished to infinity. For such versed sines are the halves of
the diagonals mentioned in Cor. 3.

Cor. 5. And therefore those forces are to the force of gravity as the said
versed sines to the versed sines perpendicular to the horizon of those para¬
bolic arcs which projectiles describe in the same time.

Cor. 6. And the same things do all hold good (by Cor. 5 of the Laws),
when the planes in which the bodies are moved, tpgether with the centres
of force which are placed in those planes, are not at rest, but move uni¬
formly forward in right lines.

PROPOSITION II. THEOREM II.

Every body that moves in any curve line described in a plane , and by a
radius , drawn to a point either immovable , or moving forward with
an uniform rectilinear motion :, describes about that point areas propor¬
tional to the times , is urged by a centripetal force directed to that point
Case. 1. For every body
that moves in a curve line,
is (by Law 1) turned aside
from its rectilinear course
by the action of some force
that impels it. And that force
by which the body is turned
off from its rectilinear course,
and is made to describe, in
equal times, the equal least
triangles SAB, SBC, SCD,

&c., about the immovable
point S (by Prop. XL. Book
1, Elem. and Law II), acts
in the place B, according to
the direction of a line par-

1U6 THE MATHEMATICAL PRINCIPLES [BOOK I.

allel 1( cC. that is, in the direction of the line BS. and in the place C,
accordii g to the direction of a line parallel to dD, that is, in the direction
of the line CS, (fee.; and therefore acts always in the direction of lines
tending to the immovable point S. Q.E.I).

Case. 2. And (by Cor. 5 of the Laws) it is indifferent whether the su-
perfices in which a body describes a curvilinear figure be quiescent, or moves
together with the body, the figure described, and its point S, uniformly
forward in right lines.

Cor. 1. In non-resisting spaces or mediums, if the areas are not propor¬
tional to the times, the forces are not directed to the point in which the
radii meet; but deviate therefrom in consequential or towards the parts to
which the motion is directed, if the description of the areas is accelerated;
but in antecedentia, if retarded.

Cor. 2. And even in resisting mediums, if the description of the areas
is accelerated, the directions of the forces deviate from the point in which
the radii meet, towards the parts to which the motion tends.

SCHOLIUM.

A body may be urged by a centripetal force compounded of several
forces; in which case the meaning of the Proposition is, that the force
which results out of all tends to the point S. But if any force acts per¬
petually in the direction of lines perpendicular to the described surface,
this force will make the body to deviate from the plane of its motion : but
will neither augment nor diminish the quantity of the described surface,
and is therefore to be neglected in the composition of forces.

PROPOSITION III. THEOREM III.

Every body , that by a radius drawn to the centre of another body, how¬
soever moved , describes areas about that centre proportional to the times ,
is urged by a force compounded out of the centripetal force ■ending to
that other body , and of all the accelerative force by which that other
body is impelled.

Let L represent the one, and T the other body; and (by Cor. 6 of the Laws)
if both bodies are urged in the direction of parallel lines, by a new force
equal and contrary to that by which the second body T is urged, the first
body L will go on to describe about the other body T the same areas as
before: but the force by which that other body T was urged will be now
destroyed by an equal and contrary force; and therefore (by Law I.) that
other body T, now left to itself, will either rest, or move uniformly forward
in a right line: and the first body L impelled by the difference of the
forces, that is, by the force remaining, will go on to describe about the other
body T areas proportional to the times. And therefore (by Theor. II.) the
difference ;f the forces is directed to the other body T as its centre. Q,.E.D

Sec. II.]

OF NATURAL PHILOSOPHY.

107

Cor. 1. Hence if the one body L, by a radius drawn to the other body T,
describ.es areas proportional to the times; and from the whole force, by which
the first body L is urged (whether that force is simple, or, according to
Cor. 2 of the Laws, compounded out of several forces), we subduct (by the
same Cor.) that whole accelerative force by which the other body is urged;
the wlio.e remaining force by which the first body is urged will tend to the
(ther body T, as its centre.

Cor. 2. And, if these areas are proportional to the times nearly, the re¬
maining force will tend to the other body T nearly.

Cor. 3. And vice versa , if the remaining force tends nearly to the other
body T, those areas will be nearly proportional to the times.

Cor. 4. If the body L, by a radius drawn to the other body T, describes
areas, which, compared with the times, are very unequal; and that other
body T be either at rest, or moves uniformly forward in a right line : the
action of the centripetal force tending to that other body T is either none
at all, or it is mixed and compounded with very powerful actions of other
forces: and the whole force compounded of them all, if they are many, is
directed to another (immovable or moveable) centre. The same thing ob¬
tains, when the other body is moved by any motion whatsoever; provided
that centripetal force is taken, which remains after subducting that whole
force acting upon that other body T.

SCHOLIUM.

Because the equable description of areas indicates that a centre is re¬
spected by that force with which the body is most affected, and by which it
is drawn back from its rectilinear motion, and retained in its orbit; why
may we not be allowed, in the following discourse, to use the equable de¬
scription of areas as an indication of a centre, about which all circular
motion is performed in free spaces ?

PROPOSITION IV. THEOREM IY.

The centripetal forces of bodies , which by equable motions describe differ -
ent circles, tend to the centres of the same circles ; and are one to the
other as the squares of the arcs described in equal t imes applied to the
radii of the circles.

These forces tend to the centres of the circles (by Prop. II., and Cor. 2,
Prop. I.), and are one to another as the versed sines of the least arcs de¬
scribed in equal times (by Cor. 4, Prop. I.); that is, as the squares of the
same arcs applied to the diameters of the circles (by Lem. VII.); and there¬
fore since those arcs are as arcs described in any equal times, and the dia-
me'ers a«re as the radii, the forces will be as the squares of any arcs de-
scr bed in the same time applied to the radii of the circles. Q.E.D.

3or. 1. Therefore, since those arcs are as the velocities of the bodies

THE MATHEMATICAL PRINCIPLES

l OS

[Book .

the centripetal forces are in a ratio compounded of the duplicate ratio of
the velocities directly, and of the simple ratio of the radii inversely.

Cor. 2. And since the periodic times are in a ratio compounded of the
ratio of the radii directly, and the ratio of the velocities inversely, the cen¬
tripetal forces, are in a ratio compounded of the ra,tio of the radii directly,
and the duplicate ratio of the periodic times inversely.

Cor. 3. Whence if the periodic times are equal, and the velocities
therefore as the radii, the centripetal forces will be also as the radii ; and
the contrary.

Cor. 4. If the periodic times and the velocities are both in the subdu-
plfcate ratio of the radii, the centripetal forces will be equal among them¬
selves ; and the contrary.'

Cor. 5. If the periodic times are as the radii, and therefore the veloci¬
ties equal, the centripetal forces will be reciprocally as the radii; and the
contrary.

Cor. 6. If the periodic times are in the sesquiplicate ratio of the radii,
and therefore the velocities reciprocally in the subduplicate ratio of the
radii, the centripetal forces will be in the duplicate ratio of the radii in¬
versely ; and the contrary.

Cor. 7. And universally, if the periodic time is as any power R n of the
radius R, and therefore the velocity reciprocally as the power R n — 1 of
the radius, the centripetal force will be reciprocally as the power R 2n 1 of
the radius; and the contrary.

Cor. 8. The same things all hold concerning the times, the velocities,
and forces by which bodies describe the similar parts of any similar figures
that have their centres in a similar position with those figures ; as appears
by applying the demonstration of the preceding cases to those. And the
application is easy, by only substituting the equable description of areas in
the place of equable motion, and using the distances of the bodies from the
centres instead of the radii.

Cor. 9. From the same demonstration it likewise follows, that the arc
which a body, uniformly revolving in a circle by means of a given centri¬
petal force, describes in any time, is a mean proportional between the
diameter of the circle, and the space which the same body falling by the
same given force would descend through in the same given time.

SCHOLIUM.

The case of the 6th Corollary obtains in the celestial bodies (as Sir
Christopher Wren, Dr. Hooke, and Dr. Halley have severally observed );
and therefore in what follows, I intend to treat more at large of those
things which relate to centripetal force decreasing in a duplicate ratio
of the distances from the centres.

Moreover, by means of the preceding Proposition and its Corollaries, we

Sec. II.]

OF NATURAL PHILOSOPHY.

109

may discover the proportion of a centripetal force to any other known
force, such as that of gravity. For if a body by means of its gravity re¬
volves in a circle concentric to the earth, this gravity is the centripetal
force of that body. But from the descent of heavy bodies, the time of one
entire revolution, as well as the arc described in any given time, is given
(by Cor. 9 of this Prop.). And by such propositions, Mr. Huygens, in his
excellent book De IJorologio Oscillatorio, has compared the force of
gravity with the centrifugal forces of revolving bodies.

The preceding Proposition may be likewise demonstrated after this
manner. In any circle suppose a polygon to be inscribed of any number
of sides. And if a body, moved with a given velocity along the sides of the
polygon, is reflected from the circle at the several angular points, the force,
with which at every reflection it strikes the circle, will be as its velocity :
and therefore the sum of the forces, in a given time, will be as that ve¬
locity and the number of reflections conjunctly; that is (if the species of
the polygon be given), as the length described in that given time, and in¬
creased or diminished in the ratio of the same length to the radius of the
circle; that is, as the square of that length applied to the radius; and
therefore the polygon, by having its sides diminished in'infinitum, coin¬
cides with the circle, as the square of the arc described in a given time ap¬
plied to the radius. This is the centrifugal force, with which the body
impels the circle; and to which the contrary force, wherewith the circle
continually repels the body towards the centre, is equal.

PROPOSITION Y. PROBLEM I.

There being given , in any places, the velocity with which a body de¬
scribes a given figure, by means of forces directed to some common

centre: to find that centre.

Let the three right lines PT, TOY, YR
touch the figure described in as many points,

P, Q,, R, and meet in T and Y. On the tan¬
gents erect the perpendiculars PA, Q.B, RC,
reciprocally proportional to the velocities of the
body in the points P, Q, R, from which the
perpendiculars were raised; that is, so that PA
may be to OB as the velocity in Q, to the velocity in P, and QB to RC
as the velocity in R to the velocity in Q. Through the ends A, B, C, of
the perpendiculars draw AD, DBE, EC, at right angles, meeting in D and
E: and the right lines TD, YE produced, will meet in S, the centre re¬
quired.

For the perpendiculars let fall from the centre S on the tangents PT,
QT, are reciprocally as the velocities of the bodies in the points P and Q

110

THE MATHEMATICAL PRINCIPLES

[Book 1

(by Cor. 1, Prop. I.), and therefore, by construction, as the perpendiculars
AP, BQ directly; that is, as the perpendiculars let fall from the point D
on the tangents. Whence it is easy to infer that the points S, D, T, are
in one right line. And by the like argument the points S, E, V are also
in one right line; and therefore the centre S is in the point where the
right lines TD, YE meet. Q.E.D.

PROPOSITION YI. THEOREM Y.

In a space void of resistance, if a body revolves in any orbit about an im¬
movable centre, and in the least time describes any arc just then na¬
scent ; and the versed sine of that arc is supposed to be drawn bisect¬
ing the chord, and produced passing through the centre of force: the
centripetal force in the middle of the arc will be as the versed sine di¬
rectly and the square of the time inversely.

For the versed sine in a given time is as the force (by Cor. 4, Prop. 1);
and augmenting the time in any ratio, because the arc will be augmented
in the same ratio, the versed sine will be augmented in the duplicate of
that ratio (by Cor. 2 and 3, Lem. XI.), and therefore is as the force and the
square of the time. Subduct on both sides the duplicate ratio of the.
time, and the force will be as the versed sine directly, and the square of
the time inversely. Q.E.D.

And the same thing may also be easily demonstrated by Corol. 4,
Lem. X.

Cor. 1. If a body P revolving about the
centre S describes a curve line APQ, which a
right line ZPR touches in any point P; and
from any other point Q of the curve, OR is / /

drawn parallel to the distance SP, meeting j
the tangent in R ; and QT is drawn perpen- s
dicular to the distance SP; the centripetal force will be reciprocally as the
SP 2 X QT 2

solid-— _ -, if the solid be taken of that magnitude which it ulti-

QR ’ °

mately acquires when the points P and Q coincide. For QR is equal to

the versed sine of double the arc QP, whose middle is P: and double the

triangle SQ,P, or SP X QT is proportional to the time in which that

double arc is described; and therefore may be used for the exponent of

the time.

Cor. 2. By a like reasoning, the centripetal force is reciprocally as the
SY 2 X QP 2

solid--; if SY is a perpendicular from the centre of force on

hr

PR the tangent of the orbit. For the rectangles SY X QP and SP X QT
are equal.

OF NATURAL PHILOSOPHY.

Ill

Sec. IT.]

Cor. 3. If the orbit is either a circle, or touches or cuts a circle c< ncen-
trieally, that is, contains with a circle the least angle of contact or sec¬
tion, havirfc the same curvature rnd the same radius of curvature at the
point P ; and if PV be a chord of this circle, drawn from the body through
the centre of force; the centripetal force will be reciprocally as the solid

SY 2 X PV.

F ° r py is^y.

Cor. 4. The same things being supposed, the centripetal force is as the
square of the velocity directly, and that chord inversely. For the velocity
is reciprocally as the perpendicular SY, by Cor. 1. Prop. I.

Cor. 5. Hence if any curvilinear figure APQ is given, and therein a
point S is also given, to which a centripetal force is perpetually directed,
that law of centripetal force may be found, by which the body P will be
continually drawn back from a rectilinear course, and. being detained in
the perimeter of that figure, will describe the same by a perpetual revolu-

SP 2 x QT 2

tion. That is, we are to find, by computation, either the solid- ^7 -

or the solid SY 2 X PV, reciprocally proportional to this force. Example:
of this we shall give in the following Problems.

PROPOSITION VII. PROBLEM II.

Jf a body revolves in the circumference of a circle; it is proposed to find
the law of centripetal force directed to any given point.

Let VQPA be the circumference of the
circle; S the given point to which as to
a centre the force tends; P the body mov¬
ing in the circumference; Q the next
place into which it is to move; and PRZ
the tangent of the circle at the preceding
place. Through the point S draw the
chord PV, and the diameter VA of the
circle: join AP, and draw QT perpen¬
dicular to SP, which produced, may meet
the tangent PR in Z; and lastly, through
the point Q, draw LR parallel to SP, meeting the circle in L, and the
tangent PZ in R. And, because of the similar triangles ZQR, ZTP,

VP A, we shall have
QRL X

therefore

AV 2

RP 2 , that is, QRL to QT 2 as AV 2 to PV 2 . And

PV 2 . SP 2

-is equal to QT 2 . Multiply those equals by

and the points P and Q coinciding, for RL write PV; then we shall have
SP 2 X PV* SP 2 X QT 2

. And therefore fhy For 1 and 5. Prop. VI.)

AV J

QR

112

THE MATHEMATICAL PRINCIPLES

[Book I.

SP 2 X PV 3

the centripetal force is reciprocally as - AyT —j that is (because AV 2

is given), reciprocally as the square of the distance or altitude SP, and the
cube of the chord PY conjunctly. Q.E.I.

The same otherwise.

On the tangent PR produced let fall the perpendicular SY; and (be¬
cause of the similar triangles SYP, VP A), we shall have AY to PY as SP
SP X PY SP 2 v PV 3

to SY, and therefore--jy-= SY, and- j-y- - = SY" 2 X PY.

And therefore (by Corol. 3 and 5, Prop. YI), the centripetal force is recip-
SP 2 X PY 3

rocally as-—> that i s (because AY is given), reciprocally as SP 2

X PY 3 . Q.E.I.

Cor. 1. Hence if the given point S, to which the centripetal force al¬
ways tends, is placed in the circumference of the circle, as at Y, the cen¬
tripetal force will be reciprocally as the quadrato-cube (or fifth power) of
the altitude SP.

Cor. 2. The force by which the body P in the
circle APTY revolves about the centre of force S T
is to the force by which the same body P may re¬
volve in the same circle, and in the same periodic
time, about any other centre of force R, as RP 2 X
SP to the cube of the right line SG, which from
the first centre of force S is drawn parallel to the
distance PR of the body from the second centre of force R, meeting the
tangent PG of the orbit in G. For by the construction of this Proposition,
the former force is to the latter as RP 2 X PT 3 to SP 2 X PV 3 ; that is, as
SP 3 X PY 3

SP X RP 2 to-pp—; or (because of the similar triangles PSG, TPV)

to SG 3 .

Cor. 3. The force by which the body P in any orbit revolves about the
centre of force S, is to the force by which the same body may revolve in
the same orbit, and the same periodic time, about any other centre of force
R. as the solid SP X RP 2 , contained under the distance of the body from
the first centre of force S, and the square of its distance from the sec¬
ond centre of force R, to the cube of the right line SG, drawn from the
first centre of the force S, parallel to the distance RP of the body from
tt *3 second centre of force R, meeting the tangent PG of the orbit in G.
For the force in this orbit at any point P is the same as in a circle of the
same curvature.

Sec. II.]

OF NATURAL PHILOSOPHY.

113

PROPOSITION VIII. PROBLEM III.

If a body mtves in the semi-circv inference PQA; it is proposed to find
the law of the centripetal force tending to a point S, so remote , that all
the lines PS. RS drawn thereto , may be taken for parallels.

From C, the centre of the semi-circle, let
the semi-diameter CA he drawn, cutting the
parallels at right angles in M and N, and
join CP. Because of the similar triangles
CPM, PZT, and RZQ, we shall have CP 2
to PM 2 as PR 2 to QT 2 ; and, from the na¬
ture of the circle, PR 2 is equal to the rect¬
angle QR X RN + ON, or, the points P, Q coinciding, to the rectangle
QR x 2PM. Therefore CP 2 is to PM 2 as QR X 2PM to QT 2 ; and
QT 2 2PM 3 , QT 2 X SP 2 2PM 3 X SP 2 , - , _ n

(JrT = " op 2 > and -QR- = -CP 2 -* And therefore ( b ?

Corol. 1 and 5, Prop. YI.), the centripetal force is reciprocally as

2PM 3 X SP 2 2SP 2

--; that is (neglecting the given ratio 'gpr)? reciprocally as

PM 3 . Q.E.I.

And the same thing is likewise easily inferred from the preceding Pro
position.

zi

K

A \

\c |

L

Q

M

V

SCHOLIUM.

And by a like reasoning, a body will be moved m an ellipsis, or even in
an hyperbola, or parabola, by a centripetal force which is reciprocally ae
the cube of the ordinate directed to an infinitely remote centre of force.

PROPOSITION IX. PROBLEM IY.

Tf a body revolves in a spiral PQS, cutting all the radii SP, SQ, cJ*c.,
in a given angle ; it is proposed to find the law of the centripetal force
tending to the centre of that spiral.

Suppose the inde¬
finitely small angle
PSQ to be given; be¬
cause, then, all the
angles are given, the

figure SPRQT will ,___,_

be given in specie. v

QT QT 2

Therefore the ratio—is also given, and p — is as QT, that is (be

QR

QR

cause the figure is given in specie), as SP. But if the angle PSQ is any
way changed, the right line QR, subtending the angle of contact QPR

8

THE MATHEMATICAL PRINCIPLES

114

[Book i

(by Lemma XI) will be changed in the duplicate ratio of PR or Q.T

QT 2

Therefore the ratio remains the same as before, that is, as SP. And

QT 2 X SP 2
OR

is as SP 3 , and therefore (by Corol. 1 and 5, Prop. VI) the

centripetal force is reciprocally as the cube of the distance SP. Q,.E.I.

The same otherwise.

The perpendicular SY let fall upon the tangent, and the chord PV of
the circle concentrically cutting the spiral, are in given ratios to the height
SP; and therefore SP 3 is as SY 2 X PV, that is (by Corol. 3 and 5, Prop.
VI) reciprocally as the centripetal force.

LEMMA XII.

All parallelograms circumscribed about any conjugate diameters of a
given ellipsis or hyperbola are equal among themselves.

This is demonstrated by the writers on the conic sections.

PROPOSITION X. PROBLEM V.

[f a body revolves in an ellipsis ; it is proposed to find the law of the

centripetal force tending to the centre of the ellipsis.

Suppose CA, CB to
be semi-axes of the
ellipsis; GP, DK, con- ^

jugate diameters; PF,

Q,T perpendiculars to
those diameters; Q,van
ordinate to the diame¬
ter GP; and if the
parallelogram Q^PR
be completed, then (by
the properties of the
oonic sections) the rec¬
tangle FvG will be to
dv 2 as PC 2 to CD 2 ;
and (because of the
similar triangles dvT, PCF), dv 2 to QT 2 as PC 2 to PF 2 ; and, by com
position, the ratio of PtfG to QT 2 is compounded of the ratio of PC 2 1<

CD 2 , and of the ratio of PC 2 to PF 2 , that is, vG

QT 2
to — as PC :
Fv

to—

CD 2 X PF 2

PC 2

Put Q,R for Fv , and (by Lem. XII) BC X CA for CD

x: PF; also (the points P and d coinciding) 2PC for vG) and multiply-

Sec. II.]

OF NATURAL PHILOSOPHY.

115

QT 2 x PC 2

ing the extremes and means together, we shall have-pr—--equal to

(ollt

2BC 2 X CA 2

--. Therefore (by Cor. 5, Prop. VI), the centripetal foroe is

2BC 2 X CA 2

reciprocally as--; that is (because 2BC 2 X CA 2 is given), re¬

ciprocally as- — y; that is, directly as the distance PC. QJEI.

The same otherwise.

In the right line PG on the other side of the point T, take the point u
so that T u may be equal to Tv ; then take uY, such as shall be to vG as
DC 2 to PC 2 . And because Qv 2 is to PvG as DC 2 to PC 2 (by the conic
sections), we shall have Q,v 2 ~ Pr X uY. Add the rectangle i/Tv to both
sides, and the square of the chord of the arc PQ, will be equal to the rect¬
angle YPv; and therefore a circle which touches the conic section in P,
and passes through the point Q,, will pass also through the point V. Now
let the points P and Q, meet, and the ratio of uY to rG, which is the same
with the ratio of DC 2 to PC 2 , will become the ratio of PV to PG, or PV

2DC 2

to 2PC: and therefore PY will be equal to pQ —• And therefore the

force by which the body P revolves in the ellipsis will be reciprocally as
2 DC 2

——X PF' 2 (by Cor. 3, Prop. YI); that is (because 2DC 2 X PF 2 is
given) directly as PC. Q.E.I.

Cor. 1. And therefore the force is as the distance of the body from the
centre of the ellipsis; and, vice versa , if the force is as the distance, the
body will move in an ellipsis whose centre coincides with the centre of force,
or perhaps in a circle into which the ellipsis may degenerate.

Cor. 2. And the periodic times of the revolutions made in all ellipses
whatsoever about the same centre will be equal. For those times in sim¬
ilar ellipses will be equal (bv Corol. 3 and S, Prop. IY); but in ellipses
that have their greater axis common, they are one to another as the whole
areas of the ellipses directly, and the parts of the areas described in the
same time inversely; that is, as the lesser axes directly, and the velocities
of the bodies in their principal vertices inversely; :hat is, as those lesser
axes directly, and the ordinates to the same point f the common axes in¬
versely ; and therefore (because of the equality of the direct and inverse
ratios) in the ratio of equality.

SCHOLIUM.

If the ellipsis, by having its centre removed to an infinite distance, de¬
generates into a parabola, the body will move in this parabola; and the

116

THE MATHEMATICAL PRINCIPLES

[Book I.

force, now tending to a centre infinitely remote, will become equable.
Which is Galileo's theorem. And if the parabolic section of the cone (by
changing the inclination of the cutting plane to the cone) degenerates into
an hyperbola, the body will move in the perimeter of this hyperbola, hav¬
ing its centripetal force changed into a centrifugal force. And in like
manner as in the circle, or in the ellipsis, if the forces are directed to the
centre of the figure placed in the abscissa, those forces by increasing or di¬
minishing the ordinates in any given ratio, or even by changing the angle
of the inclination of the ordinates to the abscissa, are always augmented
or diminished in the ratio of the distances from the centre; provided the
periodic times remain equal; so also in all figures whatsoever, if the ordi¬
nates are augmented or diminished in any given ratio, or their inclination
is any way changed, the periodic time remaining the same, the forces di¬
rected to any centre placed in the abscissa are in the several ordinates
augmented or diminished in the ratio of the distances from the centre

SECTION III.

Of the motion of bodies in eccentric conic sections.

PROPOSITION XI. PROBLEM VI.

If a body revolves in an ellipsis ; it is required to find the law of the
centripetal force tending to the focus of the ellipsis.

Let S be the focus
of the ellipsis. Draw
SP cutting the diame¬
ter DK of the ellipsis
in E, and the ordinate
in x ; and com¬
plete the parallelogram
Q,.rPR. It is evident
that EP is equal to the
greater semi-axis AC:
for drawing HI frofti
the other focus H of
the ellipsis parallel to
EC, because CS, CH
are equal, ES, El will
be also equal; so that EP is the half sum of PS, PI, that is (because of
the parallels HI, PR, and the equal angles IPR, HPZ), of PS, PH, which
taken together are equal to the whole axis 2AC. Draw Q,T perpendicu¬
lar to SP, and putting L for the princi al latus rectum of the ellipsis (or foT

Sec. III.]

OF NATURAL PHILOSOPHY.

117

2BC 2

we shall have L X OR to L X Py as QR to Py, that is, as PE
AC

or AC to PC; and L X Pv to GyP as L to Gy; and GvP to Q,y 2 as PC 2
to CD 2 ; and by (Corol. 2, Lem. VII) the points Q, and P coinciding, Q,y 2
is to Qx' in the ratio of equality; and Q.y 2 or Qv 2 is to QT 2 as EP 2 to
PF 2 , that is, as CA 2 to PF 2 , or (by Lem. XII) as CD 2 to CB 2 . And com¬
pounding all those ratios together, we shall have L X QR to QT 2 as AC

X L X PC 2 X CD 2 , or 2CB 2 X PC 2 X CD 2 to PC X Gy X CD 2 X
CB 2 , or as 2PC to Gy. But the points Q and P coinciding, 2PC and Gr
are equal. And therefore the quantities L X QR and QT 2 , proportional

SP 2

to these, will be also equal. Let those equals be drawn and L

X SP 2 will become equal to

SP 2 x QT 2
Gill

And therefore (by Corol. 1 and

5, Prop. VI) the centripetal force is reciprocally as L X SP 2 , that is, re¬
ciprocally in the duplicate ratio of the distance SP. Q.E.I.

The same otherwise.

Since the force tending to the centre of the ellipsis, by which the body
P may revolve in that ellipsis, is (by Corol. 1, Prop. X.) as the distance
CP of the body from the centre C of the ellipsis; let CE be drawn paral¬
lel to the tangent PR of the ellipsis; and the force by which the same body
P may revolve about any other point S of the ellipsis, if CE and. PS in-

PE 3

tersect in E, will be as gp 2

(by Cor. 3, Prop. VII.); that is, if the point

S is the focus of the ellipsis, and therefore PE be given as SP 2 recipro¬
cally. Q.E.I.

With the same brevity with which we reduced the fifth Problem to the
parabola, and hyperbola, we might do the like here: but because of the
dignity of the Problem and its use in what follows. I shall confirm the other
cases by particular demonstrations.

PROPOSITION XII. PROBLEM VII.

Suppose a body to move in an hyperbola ; it is required to find the law oj
the centripetal force tending to the focus of that figure.

Let CA, CB be the semi-axes of the hyperbola; PG, KD other con¬
jugate diameters ; PF a perpendicular to the diameter KD ; and Qy an
ordinate to the diameter GP. Draw SP cutting the diameter DK in E,
and the ordinate Qy in x : and complete the parallelogram QRP#. It is
evident that EP is equal to the semi-transverse axis AC; for drawing
HI, from the other focus H of the hyperbola, parallel to EC, because CS,
CH are equal, ES El will be also equal; so that EP is the half difference

ns

THE MATHEMATICAL PRINCIPLES

[Boot I

.of PS, PI; that is (be¬
cause of the parallels IH,

PR, and the equal angles
I PR, HPZ), of PS, PH,
the difference of which is
equal to the whole axis
2AC. Draw QT perpen¬
dicular to SP; and put¬
ting L for the principal
latus rectum of the hy¬
perbola (that is, for

~~r ^ ? we shall have L
AC /

X OR to L X Pv as QR
to Pv, or Vx to Pv, that is
(because of the similar tri¬
angles Vxv, PEC), as PE 11
to PC, or AC to PC.

And L X Pv will be to
Gv X Pv as L to Gv;
and (by the properties of
the conic sections) the rec¬
tangle G?*P is to Qv 2 as
PC' 2 to CD 2 ; and by (Cor. 2, Lem. VII.), Qv 2 to Q# 2 , the points Q and P
coinciding, becomes a ratio of equality; and Q.v 2 or Qv 2 is to QT 2 as EP S
to PF 2 , that is, as CA 2 to PF 2 , or (by Lem. XII.) as CD 2 to CB 2 : and,
compounding all those ratios together, we shall have L X QR to QT 2 as
AC X L X PC 2 X CD 2 , or 2CB 2 X PC 2 X CD 2 to PC X Gv X CD 2
X CB 2 , or as 2PC to Gv. But the points P and Q coinciding, 2PC and
Gv are equal. And therefore the quantities L X QR and QT 2 , propor¬
tional to them, will be also equal. Let those equals be drawn into

SP 2 SP 2 X QT 2

and we shall have L X SP 2 equal to--. And therefore (by

QR’

Cor. 1. and 5, Prop. VI.) the centripetal force is reciprocally as L X SP ;
ihat is, reciprocally in the duplicate ratio of the distance SP. Q.E.I.

The same otherwise.

Find out the force tending from the centre C of the hyperbola. This will
be proportional to the distance CP. But from thence (by Cor. 3, Prop.

PE 3

VII.) the force tending to the focus S will be as -gp, thi t is, because PE
is given reciprocally as SP 2 . Q.E.I.

Sec. III.]

OF NATURAL PHILOSOPHY.

119

And the same way may it be demonstrated, that the body having its cen¬
tripetal changed into a centrifugal force, will move in the conjugate hy¬
perbola.

LEMMA XIII.

The latus rectum of a parabola belonging to any vertex is quadruple
the distance of that vertex from the focus of the jigure.

This is demonstrated by the writers on the conic sections.

LEMMA XIV.

The perpendicular, let fall from the focus of a parabola on its tangent , is
a mean proportional between the distances of the focus from the point
of contact, and from the principal vertex of the figure.

For, let AP be the parabola, S its
focus, A its principal vertex, P the
point of contact, PO an ordinate to the
principal diameter, PM the tangent
meeting the principal diameter in M.
and SN the perpendicular from the fo¬
cus on the tangent: join AN, and because of the equal lines MS and SP,
MN and NP, MA and AO, the right lines AN, OP, will be parallel; and
thence the triangle SAN will be right-angled at A, and similar to the
equal triangles SNM, SNP; therefore PS is to SN as SN to SA. Q.E.I).
Cor. 1. PS 2 is to SN 2 as PS to SA.

Cor. 2. And because SA is given, SN 2 will be as PS.

Cor. 3. And the concourse of any tangent PM, with the right line SN,
drawn from the focus per] endicular on the tangent, falls in the right line
AN that touches the parabola in the principal vertex.

PROPOSITION XIII. PROBLEM VIII.

If a body moves in the perimeter of a parabola ; it is required to find the
law of the centripetal force tending to the focus of that figure.

Retaining the construction
of the preceding Lemma, let P
be the body in the perimeter
of the parabola; and from the
place Q, into which it is next
to succeed, draw Q,R parallel
and Q,T perpendicular to SP,
as also Qv parallel to the tan¬
gent, and meeting the diame¬
ter PG in v , and the distance'

120

THE MATHEMATICAL PRINCIPLES

[Book I.

SP in x. Now, because of the similar triangles Pxv, SPM, and of the

equal sides SP, SM of the one, the sides Vx or Q,R and Pi? of the other

will be also equal. But (by the conic sections) the square of the ordinate

Q,i? is equal to the rectangle under the latus rectum and the segment Pi?

of the diameter; that is (by Lem. NIII.), to the rectangle 4PS X Pt?, or

4PS X GIR j and the points P and GJ, coinciding, the ratio of GIi? to GLf

(by Cor. 2, Lem. VII.,) becomes a ratio of equality. And therefore Q# 2 , in

this case, becomes equal to the rectangle 4PS X OR. But (because of the

similar triangles O^T, SPN), Q,# 2 is to OT 2 as PS 2 to SN 2 , that is (by

Cor. 1, Lem. XIV.), as PS to SA ; that is, as 4PS X OR to 4SA X OR,

and therefore (by Prop. IX. Lib. V., Elem.) OT 2 and 4SA X OR are

SP 2 , SP 2 X OT 2 ,

will become equal

equal. Multiply these equals by and

OR

to SP 2 X 4SA: and therefore (by Cor. 1 and 5, Prop. VI.), the centripetal
force is reciprocally as SP 2 X 4S A; that is, because 4S A is given, recipro¬
cally in the duplicate ratio of the distance SP. O.E.I.

Cor. 1. From the three last Propositions it follows, that if any body P
goes from the place P with any velocity in the direction of any right line
PR, and at the same time is urged by the action of a centripetal force that
is reciprocally proportional to the square of the distance of the places from
the centre, the body will move in one of the conic sections, having its fo¬
cus in the centre of force; and the contrary. For the focus, the point of
contact, and the position of the tangent, being given, a conic section may
be described, which at that point shall have a given curvature. But the
curvature is given from the centripetal force and velocity of the body be¬
ing given; and two orbits, mutually touching one the other, cannot be de¬
scribed by the same centripetal force and the same velocity.

Cor. 2. If the velocity with which the body goes from its place P is
such, that in any infinitely small moment of time the lineola PR may be
thereby describel; and the centripetal force such as in the same time to
move the same body through the space Q,R ; the body will move in one of

GIT 2 . .
TTrrin its

the conic sections, whose principal latus rectum is the quantity

ultimate state, when thelineolae PR, Q,R are diminished in infinitum. In
these Corollaries I consider the circle as an ellipsis; and I except the case
where the body descends to the centre in a right line.

PROPOSITION XIV. THEOREM VI.

Tf several bodies revolve about one common centre , and the centripetal
force is reciprocally in the duplicate ratio of the distance of places
from., the centre ; I say , that the principal latera recta of their orbits
are in the duplicate ratio of the areas , which the bodies by radii drawn
to the centre discribe in the same time.

121

SEC. Ill.] OF NATURAL PHILOSOl liY.

For (by Cor 2, Prop. XII'l) the latus rectum

QT 2 .

L is equal to the quantity-^g-in its ultimate

state when the points P and Q. coincide. But
the lineola Q,R in a given time is as the gen¬
erating centripetal force; that is (by supposi-

Q,T 2

tion), reciprocally as SP 2 . And therefor c - ^ -

(atlv

is as QT 2 X SP 2 ; that is, the latus rectum L is in the duplicate ratio of
the area QT X SP. Q.E.D.

Cor. Hence the whole area of the ellipsis, and the rectangle under the
axes, which is proportional to it, is in the ratio compounded of the subdu¬
plicate ratio of the latus rectum, and the ratio of the periodic time. For
the whole area is as the area QT X SP, described in a given time, mul¬
tiplied by the periodic time.

PROPOSITION XV. THEOREM VII.

The same things being supposed , 7 say, that the periodic times in ellip¬
ses are in the sesquiplicate ratio of their greater axes.

For the lesser axis is a mean proportional between the greater axis and
the latus rectum; and, therefore, the rectangle under the axes is in the
ratio compounded of the subduplicate ratio of the latus rectum and the
sesquiplicate ratio of the greater axis. But this rectangle (by Cor. o.
Prop. XIV) is in a ratio compounded of the subduplicate ratio of the
latus rectum, and the ratio of the periodic time. Subduct from both sides
the subduplicate ratio of the latus rectum, and there will remain the ses¬
quiplicate ratio of the greater axis, equal to the ratio of the periodic time.
Q.E.D.

Cor. Therefore the periodic times in ellipses are the same as in circles
whose diameters are equal to the greater axes of the ellipses.

PROPOSITION XVI. THEOREM VIII.

The same things being supposed , and right lines being drawn to the
bodies that shall touch the orbits , and perpendiculars being let fall on
those tangents from the common focus; Isay , that the velocities oj
the bodies are in a ratio compounded of the ratio of the perpendiculars
inversely , and the subduplicate ratio of the principal latera recta
directly.

From the focus S draw SY perpendicular to the tangent PR, and the
velocity of the body P will be reciprocally in the subduplicate ratio of the
SY 2

quantity —j—. For that velocity is as the infinitely small arc PQ de*

122

THE MATHEMATICAL PRINCIPLES

[Book I.

scribed in a given moment of time, that is (by
Lem. VII), as the tangent PR; that is (because
of the proportionals PR to QT, and SP to

and SP X QT directly; but SP X QT is as
the area described in the given time, that is (by
Prop. XIV), in the subduplicate ratio of the
latus rectum. Q.E.D.

Cor. 1. The principal latera recta are in a ratio compounded of the
duplicate ratio of the perpendiculars and the duplicate ratio of the ve¬
locities.

Cor. 2. The velocities of bodies, in their greatest and least distances from
the common focus, are in the ratio compounded of the ratio of the distan¬
ces inversely, and the subduplicate ratio of the principal latera recta di¬
rectly. For those perpendiculars are now the distances.

Cor. 3. And therefore the velocity in a conic section, at its greatest or
least distance from the focus, is to the velocity in a circle, at the same dis¬
tance from the centre, in the subduplicate ratio of the principal latus rec¬
tum to the double of that distance.

Cor. 4. The velocities of the bodies revolving in ellipses, at their mean
distances from the common focus, are the same as those of bodies revolving
in circles, at the same distances ; that is (by Cor. 6, Prop. IV), recipro¬
cally in the subduplicate ratio of the distances. For the perpendiculars
are now the lesser semi-axes, and these are as mean proportionals between
the distances and the latera recta. Let this ratio inversely be compounded
with the subduplicate ratio of the latera recta directly, and we shall have
the subduplicate ratio of the distance inversely.

Cor. 5. In the same figure, or even in different figures, whose principal
latera recta are equal, the velocity of a body is reciprocally as the perpen¬
dicular let fall from the focus on the tangent.

Cor. 6. In a parabola, the velocity is reciprocally in the subduplicate
ratio^of the distance of the body from the focus of the figure; it is more
variable in the ellipsis, and less in the hyperbola, than according to this
ratio. For (by Cor. 2, Lem. XIV) the perpendicular let fall from the
focus on the tangent of a parabola is in the subduplicate ratio of the dis¬
tance. In the hyperbola the perpendicular i3 less variable; in the ellipsis
more.

Cor. 7. In a parabola, the velocity of a body at any distance from the
focus is to the velocity of a body revolving in a circle, at the same distance
from the centre, in the subduplicate ratio of the number 2 to 1; in the
ellipsis it is less, and in the hyperbola greater, than according to this ratio,
For (by Cor. 2 of this Prop.) the velocitv at the vertex of a parabola is i e

SY), as

SP X QT

or as SY reciprocally

Sec. III.]

CP NATURAL PHILOSOPHY.

123

this ratio, and (by Cor. 6 of this Prop, and Prop. IV) the same proportion
holds in all distances. And hence, also, in a parabola, the velocity is
everywhere equal to the velocity of a body revolving in a circle at half the
distance; in the ellipsis it is less, and in the hyperbola greater.

Cor. S. The velocity of a body revolving in any conic section is to the
velocity of a body revolving in a circle, at the distance of half the princi¬
pal latus rectum of the section, as that distance to the perpendicular let
fall from the focus on the tangent of the section. This appears from
Cor. 5.

Cor. 9. Wherefore since (by Cor. 6, Prop. IV), the velocity of a body
revolving in this circle is to the velocity of another body revolving in any
other circle reciprocally in the subduplicate ratio of the distances; there¬
fore, ex cequo, the velocity of a body revolving •in a conic section will be
to the velocity of a body revolving in a circle at the same distance as a
mean proportional between that common distance, and half the principal
latus rectum of the section, to the perpendicular let fall from the common
focus upon the tangent of the section.

PROPOSITION XVII. PROBLEM IX.

Supposing the centripetal force to be reciprocally proportional to the
squares of the distances of places from the centre, and that the abso¬
lute quantity of that force is knoiun ; it is required to determine the
line which a body will describe that is let go from a given place with a
given velocity in the direction of a given right line.

Let the centripetal force
tending to the point S be
such as will make the body
p revolve in any given orbit
pq ; and suppose the velocity
of this body in the place p
is known. Then from the
place P suppose the body P
to be let go with a given ve¬
locity in the direction of the
line PR; but by virtue of a
centripetal force to be immediately turned aside from that right line into
the conic section PQ,. This, the right line PR will therefore touch in P.
Suppose likewise that the right line pr touches the orbit pq in p ; and if
from S you suppose perpendiculars let fall on those tangents, the principal
latus rectum of the conic section (by Cor. 1, Prop. XVI) will be to the
principal latus rectum of that orbit in a ratio compounded of the duplicate
ratio of the perpendiculars, and the duplicate ratio of the velocities; and
is therefore given. Let this latus rectum be L ; the focus S of the conic

L24

THE MATHEMATICAL PRINCIPLES

[Book 1.

section is also given. Let the angle RPH be the complement of the angle
RPS to two right; and the line PH, in which the other focus H is placed,
is given by position. Let fall SK perpendicular on PIf, and erect the
conjugate semi-axis BC ; this done, we shall have SP 2 — 2KPH + PH 2
= SH 2 = 4CH 2 = 4BH 2 — 4BC 2 = SP + PH 2 — L X SP"+"PH =
SP 2 + 2SPH + PH 2 —L x SP“+PH. Add on both sides 2KPH —
SP 2 —PH 2 + L X SF+T?H, and we shall have L X SP~+“PH = 2SPH
•f 2KPH, or SP + PH to PH, as 2SP + 2KP to L. Whence PH is
given both in length and position. That is, if the velocity of the body
in P is such that the latus rectum L is less than 2SP + 2KP, PH will
lie on the same side of the tangent PR with the line SP; and therefore
the figure will be an ellipsis, which from the given foci S, H, and the
principal axis SP + PH, is given also. But if the velocity of the body
is so great, that the latus rectum L becomes equal to 2SP + 2KP, the
length PH will be infinite; and therefore, the figure will be a parabola,
which has its axis SH parallel to the line PK, and is thence given. But
if the body goes from its place P with a yet greater velocity, the length
PH is to be taken on the other side the tangent; and so the tangent pas¬
sing between the foci, the figure will be an hyperbola having its principal
axis equal to the difference of the lines SP and PH, and thence is given.
For if the body, in these cases, revolves in a conic section so found, it is
demonstrated in Prop. XI, XII, and XIII, that the centripetal force will
be reciprocally as the square of the distance of the body from the centre
of force S; and therefore we have rightly determined the line PQ,, which
a body let go from a given place P with a given velocity, and in the di¬
rection of the right line PR given by position, would describe with such a
force. Q.E.F.

Cor. 1. Hence in every conic section, from the principal vertex D, the
latus rectum L, and the focus S given, the other focus H is given, by
taking DH to DS as the latus rectum to the difference between the latus
rectum and 4DS. For the proportion, SP + PH to PH as 2SP + 2KP
to L, becomes, in the case of this Corollary, DS + DH to DH as 4DS to
L, and by division DS to DH as 4DS — L to L.

Cor. 2. Whence if the velocity of a body in the principal vertex D is
given, the orbit may be readily found; to wit, by taking its latus rectum
to twice the distance DS, in the duplicate ratio of this given velocity to
the velocity of a body revolving in a circle at the distance DS (by Cor.
3, Prop. XVI.), and then taking DH to DS as the latus rectum to the
difference between the latus rectum and 4DS.

Cor. 3. Hence also if a body move in any conic section, and is forced
out of its orbit by any impulse, you may discover the orbit in which it will
afterwards pursue its tourse. For bv compounding the proper motion oi

OP NATURAL PHILOSOPHY.

125

Sec. IV.]

the body with that motion, which the impulse alone would generate, you
will have the motion with which the body will go off from a given place
of impulse in the direction of a right line given in position.

Cor. 4. And if that body is continually disturbed by the action of some
foreign force, we may nearly know its course, by collecting the changes
which that force introduces in some points, and estimating the continual
changes it will undergo in the intermediate places, from the analogy that
appears in the progress of the series.

SCHOLIUM.

If a body P, by means of a centripetal
force tending to any given point R, move
in the perimeter of any given conic sec¬
tion whose centre is C; and the law of
the centripetal force is required: draw
CG parallel to the radius RP, and meet¬
ing the tangent PG of the orbit in G;
and the force required (by Cor. 1, and

Schol. Prop. X., and Cor. 3, Prop. VII.) will be as

SECTION IY.

Of the finding of elliptic, parabolic, and hyperbolic orbits, from ttu
focus given.

LEMMA XY.

Tffrom the two foci S, H, of any ellipsis or hyberbola, we draw to any
third point Y the right lines SY, HY, whereof one HY is equal to the
principal axis of the figure, that is, to the axis in which the foci are
situated, the other, SY, is bisected in T by the perpendicular TR let
fall upon it; that perpendicular TR will somewhere touch the conic
section: and, vice versa, if it does touch it, HY will be equal to the
principal axis of the figure.

For, let the perpendicular TR cut the right line
HY, produced, if need be, in R; and join SR. Be¬
cause TS, TY are equal, therefore the right lines SR,

VR, as well as the angles TRS, TRY, will be also
equal. Whence the point R will be in the conic section, and the perpen¬
dicular TR will touch the same; and the contrary. Q,.E.D.

126

THE MATHEMATICAL PBINCIP..ES

[Book 1

PROPOSITION XVIII. PROBLEM X.

From a focus and the principal axes given, to describe elliptic and hy¬
perbolic trajectories, which shall pass through given points, and touch
right lines given by position.

Let S be the common focus of the figures; AB A jj

the length of the principal axis of any trajectory; "~p

P a point through which the trajectory should \ /R

pass; and TR a right line which it should touch. / __ -^1/

About the centre P, with the interval AB —SP, ^ S yf

if the orbit is an ellipsis, or AB r SP, if the ^ G

orbit is an hyperbola, describe the circle HG. On the tangent TR let fall
the perpendicular ST, and produce the same to V, so that TV may be
equal to ST; and about V as a centre with the interval AB describe the
circle FH. In this manner, whether two points P, p, are given, or two
tangents TR, tr , or a point P and a tangent TR, we are to describe two
circles. Let H be their common intersection, and from the foci S, H, with
the given axis describe the trajectory: I say, the thing is done. For (be¬
cause PH -j- SP in the ellipsis, and PH — SP in the hyperbola, is equal
to the axis) the described trajectory will pass through the point P, and (by
the preceding Lemma) will touch the right line TR. And by the same
argument it will either pass through the two points P, p , or touch the two
right lines TR, tr. Q,.E.F.

PROPOSITION XIX. PROBLEM XI.

About a given focus, to describe a parabolic trajectory, which shall pass
through given points, and touch right lines given by position.

Let S be the focus, P a point, and TR a tangent of
the trajectory to be described. About P as a centre,
with the interval PS, describe the circle FG. From
the focus let fall ST perpendicular on the tangent, and
produce the same to V, so as TV may be equal to ST.

After the same manner another circle fg is to be de¬
scribed, if another pointy) is given ; or another point vl|
is to be found, if another tangent tr is given; then draw
the right line IF, which shall touch the two circles FG, fg, if two points
P, p are given; or pass through the two points V, v, if two tangents TR,
tr, are given: or touch the circle FG, and pass through the point V, if the
point P and the tangent TR are given. On FI let fall the perpendicular
SI, and bisect the same in K; and with the axis SK and principal vertex K
describe a parabola : I say the thing is done. For this parabola (because
SK is equal to IK, and SP to FP) will pass through the point P; and

Sec. IV.]

OF NATURAL PHILOSOPHY.

1 27

(by Cor. 3, Lem. XIV) because ST is equal to TV. and STR a light an¬
gle, it will touch the right line TR. Q.E.F.

PROPOSITION XX. PROBLEM XII.

About a given focus to describe any trajectory given in specie which shah

pass through given points , and touch right lines given by position.

Case 1. About the focus S it is re- ^
uired to describe a trajectory ABC, pass- f
ing through two points B, C. Because the "K-l-
trajectory is given in specie, the ratio of the j
principal axis to the distance of the foci GAS IX

will be given. In that ratio take KB to BS, and LC to CS. About the
centres B, C, with the intervals BK, CL, describe two circles; and on the
right line KL, that touches the same in K and L, let fall the perpendicu¬
lar SG; which cut in A and a , so that GA may be to AS, and Ga to aS,
as KB to BS ; and with the axis A a, and vertices A, a , describe a trajectory :
I say the thing is done. For let H be the other focus of the described
figure, and seeing GA is to AS as Ga to aS, then by division we shall
have Ga—GA, or A a to «S—AS, or SH in the same ratio, and therefore
in the ratio which the principal axis of the figure to be described has to
the distance of its foci; and therefore the described figure is of the same
species with the figure which was to be described. And since KB to BS,
and LC to CS, are in the same ratio, this figure will pass through the
points B, C, as is manifest from the conic sections.

Case 2. About the focus S it is required to v
describe a trajectory which shall somewhere
touch two right lines TR, tr. From the focus
on those tangents let fall the perpendiculars
ST, St, which produce to V, v , so that TV, tv
may be equal to 'PS, tS. Bisect Yv in O, and
erect the indefinite perpendicular OH, and cut
the right line VS infinitely produced in K and
k, so that VK be to KS, and Yk to kS, as the principal axis of the tra¬
jectory to be described is to the distance of its foci. On the diameter
K.k describe a circle cutting OH in H; and with the foci S, H, and
principal axis equal to VH, describe a trajectory : I say, the thing is done.
For bisecting Kk in X, and joining HX, HS, HV, Hv, because VK is to
KS as Yk to kS; and by composition, as VK + Yk to KS -fi kS ; and
by division, as Yk — VK to kS — KS, that is, as 2VX to 2KX, and
2KX to 2SX, and therefore as VX to HX and HX to SX, the triangles
VXH, HXS will be similar; therefore VH will be to SH as VX to XH;
and therefore as VK to KS. Wherefore VH, the principal axis of the
described trajectory, has the same ratio to SH, the distance of the foci, as

128

THE MATHEMATICAL PRINCIPLES

[Book 1.

II

""•V

R

<f X

V.T

K S k

the principal axis of the trajectory which was to be described has to the
distance of its foci; and is therefore of the same species. And seeing VH,
vH are equal to the principal axis, and VS, vS are perpendicularly bisected
by the right lines TR, tr, it is evident (by Lem. XV) that those right
lines touch the described trajectory. Q.E.F.

Case. 3. About the focus S it is required to describe a trajectory, which
shall touch a right line TR in a given Point R. On the right line TR
Jet fall the perpendicular ST, which produce to V, so that TV may be
equal to ST; join VR, and cut the right line VS indefinitely produced
in K and k, so. that VK may be to SK, and Yk to Sk, as the principal
axis of the ellipsis to be described to the distance of its foci; and on the
diameter Kk describing a circle, cut the
right line VR produced in H; then with
the foci S, H, and principal axis equal to
VH, describe a trajectory: I say, the thing
is done. Eor VH is to SH as VK to SK,
and therefore as the principal axis of the trajectory which was to be de¬
scribed to the distance of its foci (as appears from what we have demon¬
strated in Case 2); and therefore the described trajectory is of the same
species with that which was to be described; but that the right line TR,
by which the angle VRS is bisected, touches the trajectory in the point R,
is certain from the properties of the conic sections. Q.E.F.

Case 4. About the focus S it is r

required to describe a trajectory
APB that shall touch a right line
TR, and pass through any given
point P without the tangent, and
shall be similar to the figure apb,
described with the principal axis ab,
and foci s, h. On the tangent TR
let fall the perpendicular ST, which
produce to V, so that TV may be
equal to ST; and making the an¬
gles hsq, shq, equal to the angles VSP, SVP, about q as a centre, and
with an interval which shall be to ab as SP to VS, describe a circle cut¬
ting the figure apb in p : join sp, and draw
SH such that it may be to sh as SP is to sp,
and may make the angle PSH equal to the
angle psh, and the angle VSH equal to the
angle psq. Then with the foci S, H, and
principal axis AB, equal to the distance VH,
describe a conic section: I say, the thing is
done; for if sv is drawn so that it shall be to

v

OF NATURAL PHILOSOPHY.

129

Sec. IV.]

sp as sh is to sq, and shall make the angle vsp equal to the angle hsq, and
the angle vsh equal to the angle psq, the triangles svh, spq, will be similar,
and therefore vh will be to pq as sh is to sq ; that is (because of the simi¬
lar triangles V SP, hsq), as VS is to SP, or as ab to pq. Wherefore
vh and ab are equal. But, because of the similar triangles VSH. vsh, VH
is to SH as vh to sh ; that is, the axis of the conic section now described
is to the distance of its foci as the axis ab to the distance of the foci sh ;
and therefore the figure now described is similar to the figure aph. But,
because the triangle PSH is similar to the triangle psh, this figure passes
through the point P; and because YH is equal to its axis, and VS is per¬
pendicularly bisected by the right line TR, the said figure touches the
right line TR. Q.E.F-

LEMMA XYI.

From three given points to draw to a fourth point that is not given three

rigid lines whose differences shall be either given, or none at all.

Case 1. Let the given points be A, B, C, and Z the fourth point which
we are to find; because of the given difference of the lines AZ, BZ, the
locus of the point Z will be an hyperbola
whose foci are A and B, and whose princi¬
pal axis is the given difference. Let that
axis be MN. Taking PM to MA as MN
is to AB, erect PR perpendicular to AB,
and let fall ZR perpendicular to PR; then
from the nature of the hyperbola, ZR will
be to AZ as MN is to AB. And by the
like argument, the locus of the point Z will
be another hyperbola, whose foci are A, C, and whose principal axis is the
difference between AZ and CZ; and Q.S a perpendicular on AC may be
drawn, to which (QS) if from any point Z of this hyperbola a perpendicular
ZS is let fall (this ZS), shall be to AZ as the difference between AZ and
CZ is to AC. Wherefore the ratios of ZR and ZS to AZ are given, and
consequently the ratio of ZR to ZS one to the other ; and therefore if the
right lines RP, SQ, meet in T, and TZ and TA are drawn, the figure
TRZS will be given in specie, and the right line TZ, in which the point
Z is somewhere placed, will be given in position. There will be given
also the right line TA, and the angle ATZ; and because the ratios of AZ
and TZ to ZS are given, their ratio to each other is given also; and
thence will be given likewise the triangle ATZ, whose vertex is the point
Z. Q.E.I.

Case 2. If two of the three lines, for example AZ and BZ, are equal,
draw the right line TZ so as to bisect the right line AB; then find the
triangle ATZ as above. Q.E.I.

9

130 THE MATHEMATICAL PRINCIPLES [BOOK I.

Case 3. If all the three are equal, the point Z will be placed in the
centre of a circle that passes through the points A, B, C. Q.E.I.

This problematic Lemma is likewise solved in Apollonius’s Book ot
Tactions restored by Vieta.

PROPOSITION XXL PROBLEM XIII.

About a given focus to describe a trajectory that shall pass through

given points and touch right lines given by position.

Let the focus S, the point P, and the tangent TR be given, and suppose
that the other focus H is to be found.

On the tangent let fall the perpendicular
ST, which produce to Y, so that TY may
be equal to ST, and YH will be equal
to the principal axis. Join SP, HP, and
SP will be the difference between HP and
the principal axis. After this manner,
if more tangents TR are given, or more
points P, we shall always determine as
many lines YH, or PH, drawn from the said points Y or P, to the focus
H, which either shall be equal to the axes, or differ from the axes by given
lengths SP ; and therefore which shall either be equal among themselves,
or shall have given differences; from whence (by the preceding Lemma),
that other focus H is given. But having the foci and the length of the
axis (which is either YH, or, if the trajectory be an ellipsis, PH + SP;
or PH — SP, if it be an hyperbola), the trajectory is given. Q.E.I.

SCHOLIUM.

When the trajectory is an hyperbola, I do not comprehend its conjugate
hyperbola under the name of this trajectory. For a body going on with a
continued motion can never pass out of one hyperbola into its conjugate
hyperbola.

The case when three points are given
is more readily solved thus. Let B, C, ^

I), be the given points. Join BC, CD,
and produce them to E, F, so as EB may ®
be to EC as SB to SC; and FC to FD
as SC to SD. On EF drawn and pro¬
duced let fall the perpendiculars SG,

BH, and in GS produced indefinitely
take GA to AS, and Ga to aS, as HB
is to BS; then A will be the vertex, and A a the principal axis of the tra¬
jectory ; which, according as GA is greater than, equal to, or less than

OF NATURAL PHILOSOPHY.

131

Sec. V.]

AS. will be either an ellipsis, a parabola, or an hyperbola; the point a in
the first case falling on the same side of the line GF as the point A; in
the second, going oft* to an infinite distance; in the third, falling on the
other side of the line GF. For if on GF the perpendiculars Cl, DK are
let fall, IC will be to HB as EC to EB; that is, as SC to SB; and by
permutation, IC to SC as HB to SB, or as GA to SA. And, by the like
argument, we may prove that KD is to SD in the same ratio. Where¬
fore the points B, C, D lie in a conic section described about the focus S.
in such manner that all the right lines drawn from the focus S to the
several points of the section, and the perpendiculars let fall from the same
points on the right line GF, are in that given ratio.

That excellent geometer M. De la Hire has solved this Problem much
after the same way, in his Conics, Prop. XXV., Lib. VIII.

SECTION V.

How the orbits are to be found when neither focus is given.
LEMMA XVII.

Iffrom any point P of a given conic section , to the four produced sides
AB, CD, AC, DB, of any trapezium, ABDC inscribed in that section ,
as many right lines PQ, PR, PS, PT are drawn in given angle*,
each line to each side ; the rectangleVQ, X PR of those on the opposite
sides AB, CD, will be to the rectangle PS X PT of those on the other
two opposite sides AC, BD, m a given ratio.

Case 1. Let us suppose, first, that the lines drawn c

to one pair of opposite sides are parallel to either of P ?t

the other sides; as PQ, and PR to the side AC, and s j j

PS and PT to the side AB. And farther, that one I i^D

pair of the opposite sides, as AC and BD, are parallel | f°

betwixt themselves; then the right line which bisects £ - Iq —' B

those parallel sides will be one of the diameters of the K

conic section, and will likewise bisect RQ. Let O be the point in which
RQ is bisected, and PO will be an ordinate to that diameter. Produce
PO to K, so that OK may be equal to PO, and OK will be an ordinate
on the other side of that diameter. Since, therefore, the points A, B, P
and K are placed in the conic section, and PK cut 3 AB in a given angle
the rectangle PQK (by Prop. XVII., XIX., XXI. and XXIII., Book HI.,
of Apollonius's Conics) will be to the rectangle AQB in a given ratio.
But QK and PR are equal, as being the differences of the equal lines OK,
OP, and OQ, OR; whence the rectangles PQK and PQ X PR are equal ;
and therefore the rectangle PQ X PR is to the rectangle A 9 B, that is, to
the rectangle PS X PT in a given ratio. Q.E.D

132

THE MATHEMATICAL PRINCIPLES

[Book f

Case 2. Let ns next suppose that the oppo¬
site sides AC and BD of the trapezium are not
parallel. Draw Be/ parallel to AC, and meeting
as well the right line ST in Z, as the conic section
in d. Join C d cutting PQ in r, and draw DM
parallel to PQ,, cutting C d in M, and AB in N.

Then (because of the similar triangles BTZ,

DBN), B/ or PQ is to TZ as DN to NB. And
so Rr is to AQ or PS as DM to AN. Wherefore, by multiplying the antece¬
dents by the antecedents, and the consequents by the consequents, as the
rectangle PQ X Rr is to the rectangle PS X TZ, so will the rectangle
IN DM be to the rectangle ANB; and (by Case 1) so is the rectangle
PQ X Pr to the rectangle PS X PZ; and by division, so is the rectangle
PQ X PR to the rectangle PS X PT. Q.E.D.

Case 3. Let us suppose, lastly, the four lines
PQ, PR, PS, PT, not to be parallel to the sides
AC, AB, hut any way inclined to them. In their
place draw P q, Pr, parallel to AC ; and Ps, PZ
parallel to AB; and because the angles of the
triangles PQy, PRr, PSs, PTZ are given, the ra¬
tios of FQ to P q, PR to Pr, PS to Ps, PT to PZ
will he also given; and therefore the compound-
<d ratios PQ X PR to Vq X Pr, and PS X PT to

Q 9
P s

B

X PZ are

given. But from what we have demonstrated before, the ratio of P q X P?
to P 5 X PZ is given; and therefore also the ratio of PQ X PR to PS X
PT. Q.E.D.

LEMMA XVIII.

The s wit things supposed , if the rectangle PQ X PR of the lines drawn
to the two opposite sides of the trapezium is to the rectangle PS X PT
of those drawn to the other tiro sides in a given ratio , the point P,
from whence those lines are drawn , will be placed in a conic section
described about the trapezium.

Conceive a conic section to be described pas¬
sing through the points A, B, C, D, and any
one of the infinite number of points P, as for
example p ; I say, the point P will be always c
placed in this section. If you deny the thing,
join AP cutting this conic section somewhere
else, if possible, than in P, as in b. Therefore
if from those points p and b, in the given angles
to the sides of the trapezium, we draw the right
lines pq , pr , ps , pt , and bk , bn , bf, bd , we shall have, as bk X bn to bf X b(%

Sec. Y.]

OF NATURAL PHILOSOPHY

133

so (by Lem. XVII) pq X pr to ps X pt; and so (by supposition) PQ x
PR to PS X PT. And because of the similar trapezia bkAf\ PQAS, as
bk to bf, so PQ to PS. Wherefore by dividing the terms of the preceding
proportion by the correspondent terms of this, we shall have bn to bd as
PR to PT. And therefore the equiangular trapezia Thibd, DRPT, are
similar, and consequently their diagonals D6, DP do coincide. Wherefore
b falls in the intersection of the right lines AP, DP, and consequently
coincides with the point P. And therefore the point P, wherever it is
taken, falls to be in the assigned conic section. Q.E.D.

Cor. Hence if three right lines PQ, PR, PS, are drawn from a com¬
mon point P, to as many other right lines given in position, AB, CD, AC,
each to each, in as many angles respectively given, and the rectangle PQ
X PR under any two of the lines drawn be to the square of the third PS
in a given ratio; the point P, from which the right lines are drawn, will
be placed in a conic section that touches the lines AB, CD in A and C ;
and the contrary. For, the position of the three right lines AB, CD, AC
remaining the same, let the line BD approach to and coincide with the
line AC; then let the line PT come likewise to coincide with the line PS ;
and the rectangle PS X PT will become PS 2 , and the right lines AB, CD,
which before did cut the curve in the points A and B, C and D, can no
longer cut, but only touch, the curve in those coinciding points.

SCHOLIUM.

In this Lemma, the name of conic section is to be understood in a larsfe
sense, comprehending as well the rectilinear section through the vertex of
the cone, as the circular one parallel to the base. For if the pointy? hap¬
pens to be in a right line, by which the points A and D, or C and B are
joined, the conic section will be changed into two right lines, one of which
is that right line upon which the point p falls,
and the other is a right line that joins the other
two of <he four points. If the two opposite an¬
gles of the trapezium taken together are equal c
to two right angles, and if the four lines PQ,

PR, PS, PT, are drawn to the sides thereof at
right angles, or any other equal angles, and the
rectangle PQ X PR under two of the lines
drawn PQ and PR, is equal to the rectangle
PS X PT under the other two PS and PT, the conic section will become
a circle. And the same thing will happen if the four lines are drawn in
any angles, and the rectangle PQ X PR, under one pair of the lines drawn,
is to the rectangle PS X PT under the other pair as the rectangle under
the sines of the angles S, T, in which the two last lines PS, PT are drawn
to the rectangle under the sines of the angles Q, R, in which the first tw«

134 THE MATHEMATICAL PRINCIPLES [BOOK I.

PQ, PR are drawn. In all other cases the locus of the point P will be
one of the three figures which pass commonly by the name of the conic
sections. But in room of the trapezium A BCD, we may substitute a
quadrilateral figure whose two opposite sides cross one another like diago¬
nals. And one or two of the four points A, B, C, D may be supposed to
be removed to an infinite distance, by which means the sides of the figure
which converge to those points, will become parallel; and in this case the
conic section will pass through the other points, and will go the same way
as the parallels in infinitum.

LEMMA XIX.

7b find a point P from which if four right lines PQ, PR, PS, PT an
drawn to as many other right lines AB, CD, AC, BD, given by posi¬
tion , each to each , at given angles , the rectangle PQ X PR, under any
two of the lines drawn, shall be to the rectangle PS X PT, under the
other two. in a given ratio.

Suppose the lines AB, CD, to which the two
right lines PQ, PR, containing one of the rect¬
angles, are drawn to meet two other lines, given
by position, in the points A, B, C, D. From one
of those, as A, draw any right line AH, in which
you would find the point P. Let this cut the
opposite lines BD, CD, in H and I; and, because
all the angles of the figure are given, the ratio of
PQ to PA, and PA to PS, and therefore of PQ
to PS, will be also given. Subducting this ratio from the given ratio of
PQ X PR to PS X PT, the ratio of PR to PT will be given; and ad¬
ding the given ratios of PI to PR, and PT to PH, the ratio of PI to PH.
and therefore the point P will be given. Q.E.I.

Cor. 1. Hence also a tangent may be drawn to any point D of the
locus of all the points P. For the chord PD, where the points P and I)
meet, that is, where AH is drawn through the point D, becomes a tangent.
In which case the ultimate ratio of the evanescent lines IP and PH will
be found as above. Therefore draw CF parallel to AD, meeting BD in
F, and cut it in E in the same ultimate ratio, then DE will be the tan¬
gent ; because CF and the evanescent IH are parallel, and similarly cut in
E and P.

Cor. 2. Hence also the locus of all the points P may be determined.
Through any of the points A, B, C, D, as A, draw AE touching the locus,
and through any other point B parallel to the tangent, draw BF meeting
the locus in F ; and find, the point F by this Lemma. Bisect BF in G,
and, drawing the indefinite line AG, this will be the position of the dia¬
meter to which BG and FG are ordinates. Let this AG meet the locus

Sec. Y.J

OF NATURAL PHILOSOPHY.

1 35

in H, and AH will be its diameter or latus trans-
versum, to which the latus rectum will be as BG 2
to AG X GH. If AG nowhere meets the locus,
the line AH being infinite, the locus will be a par¬
abola; and its latus rectum corresponding to the
BG 2

diameter AG will be - . But if it does meet it
AG

anywhero, the locus will be an hyperbola, when
the points A and H are placed on the same side the point G; and an
ellipsis, if the point G falls between the points A and H; unless, perhaps,
the angle AGB is a right angle, and at the same time BG 2 equal to the
rectangle AGH, in which case the locus will be a circle.

And so we have given in this Corollary a solution of that famous Prob¬
lem of the ancients concerning four lines, begun by Euclid, and carried on
by Apollonius; and this not an analytical calculus, but a geometrical com¬
position, such as the ancients required.

LEMMA XX.

If the two opposite angular points A and P of any parallelogram ASPQ
touch any conic section in the points A and P ; and the sides AQ, AS
of one of those angles , indefinitely produced , meet the same conic section
in B and C ; and from the points of concourse B and C to any fifth
point D of the conic section, two right lines BD, CD are drawn meet¬
ing the two other sides PS, PQ of the parallelogram, indefinitely pro¬
duced in T and R; the parts PR and PT, cut off from the sides, will
always be one to the other in a given ratio. And vice versa, if those
parts cut off are one to the other in a given ratio, the locus of the point
D will be a, conic section passing through the four points A, B, C, P
Case 1. Join BP, CP, and from the point
D draw the two right lines DG, DE, of which
the first DG shall be parallel to AB, and
meet PB, PQ, CA in H, I, G; and the other
DE shall be parallel to AC, and meet PC ?

PS, AB, in F, K, E; and (by Lem. XVII)
the rectangle DE X DF will be to the rect¬
angle DG X DH in a given ratio. But
PQ is to DE (or IQ) as PB to HB, and con¬
sequently as PT to DH; and by permutation PQ is to PT as DE to
DH. Likewise PR is to DF as RC to DC, and therefore as (IG or) PS
to DG; and by permutation PR is to PS as DF to DG; and, by com¬
pounding those ratios, the rectangle PQ X PR will be to the rectangle
PS X PT as the rectangle DE X DF is to the rectangle DG X DH.
and consequently in*a given ratio. But PQ and PS are given, and there¬
fore the ratio of PR to PT is given. Q.E.D.

THE MATHEMATICAL PRINCIPLES

136

[Book L

Case 2. But if PR and PT are supposed to be in a given ratio one to
the other, then by going back again, by a like reasoning, it will follow
that the rectangle DE X DF is to the rectangle DG X DH in a given
ratio; and so the point D (by Lem. XVIII) will lie in a conic section pass¬
ing through the points A, B, C, P, as its locus. Q.E.D.

Cor. 1. Hence if we draw BC cutting PQ in r and in PT take Vt to
Pr in the same ratio which PT has to PR; then B£ will touch the conic
section in the point B. For suppose the point D to coalesce with the point
B, so that the chord BD vanishing, BT shall become a tangent, and CD
and BT will coincide with CB and B£.

Cor. 2. And, vice versa, if B£ is a tangent, and the lines BD, CD meet
in any point D of a conic section, PR will be to PT as Pr to P£. And,
on the contrary, if PR is to PT as Pr to Vt, then BD and CD will meet
in some point D of a conic section.

Cor. 3. One conic section cannot cut another conic section in more than
four points. For, if it is possible, let two conic sections pass through the
five points A* B, C, P, O; and let the right line BD cut them in the
points D, d, and the right line Cd cut the right line PQ, in q. Therefore
PR is to PT as P# to PT: whence PR and P q are equal one to the other,
against the supposition.

LEMMA XXI.

If two moveable and indefinite right lines BM, CM drawn through given
points B, C, as poles , do by their point of concourse M describe a third
right line MN given by position ; and other two indefinite right lines
BD,CD are drawn , making with the former two at those given paints
B, C, given angles , MBD, MCD : I say, that those two right lines BD,
CD will by their point of concourse D describe a conic section passing
through the points B, C. And, vice versa, if the right lines BD, CD
do by their point of concourse D describe a conic section passing
through the given points B, C, A, and the angle DBM is always
equal to the given angle ABC, as well as the angle DCM always
equal to the given angle AOB, the point M will lie in a right line
given by position , as its locus.

For in the right line MN let a point
N be given, and when the moveable point
M falls on the immoveable point N, let
the moveable point D fall on an immo¬
vable point P. Join CN, BN, CP, BP,
and from the point P draw the right lines
PT, PR meeting BD, CD in T~and R, C
and making the angle BPT epial to the
given angle BNM, and the angle CPR

Sec. V.J

OF NATURAL PHILOSOPHY.

137

equal to the given angle CNM. Wherefore since (by supposition) the an¬
gles MBD, NBP are equal, as also the angles MOD, NCP, take away the
angles NBD and NCD that are common, and there will remain the angles
NBM and PBT, NCM and PCR equal; and therefore the triangles NBM,
PBT are similar, as also the triangles NCM, PCR. Wherefore PT is to
NM as PB to NB ; and PR to NM as PC to NC. But the points, B, C,
N, P are immovable: wheiefore PT and PR have a given ratio to NM,
and consequently a given ratio between themselves; and therefore, (by
Lemma XX) the point D wherein the moveable right lines BT and CR
perpetually concur, will be placed in a conic section passing through the
points B, C, P. Q.E.D.

And, vice versa , if the moveable point
D lies in a conic section passing through
the given points B, C, A ; and the angle
DBM is always equal to the given an¬
gle ABC, and the angle DCM always
equal to the given angle ACB, and when
the point D falls successively on any
two immovable points p , P, of the conic C
section, the moveable point M falls suc¬
cessively on two immovable points N.

Through these points ??, N, draw the right line ??N: this line ?iN will be
the perpetual locus of that moveable point M. For, if possible, let the
point M be placed in any curve line. Therefore the point D will be placed
in a conic section passing through the five points B, C, A, p , P, when the
point M is perpetually placed in a curve line. But from what was de¬
monstrated before, the point D will be also placed in a conic section pass¬
ing through the same five points B, C, A, p , P, when the point M is per¬
petually placed in a right line. Wherefore the two conic sections will both
pass through the same five points, against Corol. 3, Lem. XX. It is
therefore absurd to suppose that the point M is placed in a curve line.

QE.D.

PROPOSITION XXII. PROBLEM XIY.

To describe a trajectory that shall pass through Jive given points .

Let the five given points be A, B, C, P, D. c
From any one of them, as A, to any other
two as B, C, which may be called the poles,
draw the right lines AB, AC, and parallel to
those the lines TPS, PRO, through the fourth
point P. Then from the two poles B, C,
draw through the fifth point D two indefinite
lines BDT, CRD, meeting with the last drawn lines TPS, PRQ (the

138

THE MATHEMATICAL PRINCIPLES

[Book L

former with the former, and the latter with the latter) in T and R. Then
drawing the right line tr parallel to TR, cutting off from the right lines
PT, PR, any segments P t, Pr, proportional to PT, PR; and if through
their extremities, t, r, and the poles B, C, the right lines B/, Cr are drawn,
meeting in d , that point d will be placed in the trajectory required. For
(by Lem. XX) that point d is placed in a conic section passing through
the four points A, B, C, P ; and the lines Rr, T t vanishing, the point d
comes to coincide with the point D. Wherefore the conic section passes
through the five points A, B, C, P, D. Q.E.D.

The same otherwise .

Of the given points join any three, as A, B,

C; and about two of them B, C, as poles,
making the angles ABC, ACB of a given
magnitude to revolve, apply the legs BA,

CA, first to the point D, then to the point P,
and mark the points M, N, in which the other
legs BL, CL intersect each other in both cases.

Draw the indefinite right line MN, and let
those moveable angles revolve about their
poles B, C, in such manner that the intersection, which is now supposed to
be m, of the legs BL, CL, or BM, CM, may always fall in that indefinite
right line MN ; and the intersection, which is now supposed to be d, of the
legs BA CA, or BD, CD, will describe the trajectory required, PADe/B.
For (by Lem. XXI) the point d will be placed in a conic section passing
through the points B, C ; and when the point rn comes to coincide with
the points L, M, N, the point d will (by construction) come to coin¬
cide with the points A, D, P. Wherefore a conic section will be described
that shall pass through the five points A, B, C, P, D. Q,.E.F.

Cor. 1. Hence a right line may be readily drawn which shall be a tan¬
gent to the trajectory in any given point B. Let the point d come to co¬
incide with the point B, and the right line B d will become the tangent
required.

Cor. 2. Hence also may be found the centres, diameters, and latera recta
of the trajectories, as in Cor. 2, Lem. XIX.

SCHOLIUM.

The former of these constructions will be- c
come something more simple by joining ,
and in that line, produced, if need be, aking
B p to BP as PR is to PT; and t rough p
draw the indefinite right ine pe parallel to S
PT, and in that line pe taking always pe
equal to P/ , and draw the right lines Be, Cr

Sec. V.J

OF NATURAL PHILOSOPHY.

139

to meet in d. For since Pr to P t, PR to PT, pE to PB, pe to P t, are all in
the same ratio, pe and P/' will be always equal. After this manner the
points of the trajectory are most readily found, unless you would rather
describe the curve mechanically, as in the second construction.

PROPOSITION XXIII. PROBLEM XV.

To describe a trajectory that shall pass through four given points , and
touch a right line given by position.

Case 1. Suppose that HB is the
given tangent, B the point of contact,
and C, L, P, the three other given
points. Jon BO. and draw PS paral¬
lel to BH, and PQ parallel to BC;
complete the parallelogram BSPQ.

Draw BD cutting SP in T, and CD
cutting PQ, in R. Lastly, draw any
line tr parallel to TR, cutting off
from PQ, PS, the segments Pr, Et proportional to PR, PT respectively ;
and draw Cr, Et their point of concourse d will (by Lem. XX) always fall
on the trajectory to be described.

The same otherwise.

1 et tie angle CBH of a given magnitude re¬
volve about the pole B, as also the rectilinear ra¬
dius PC, both ways produced, about the pole C.

Mark the points M, N, on which the leg BC of
the angle cuts that radius when BH, the other
leg thereof, meets the same radius in the points
P and D. Then drawing the indefinite line MN,
let that radius CP or CD and the leg BC of the
angle perpetually meet in this line; and the
point of concourse of the other leg BH with the
radius will delineate the trajectory required.

For if in the constructions of the preceding Problem the point A comes
to a coincidence with the point B, the lines CA and CB will coincide, and
the line AB, in its last situation, will become the tangent BH ; and there*
fore the constructions there set down will become the same with the con¬
structions here described. Wherefore the concourse of the leg BH with
the radius will describe a conic section passing through the points C, D,
P, and touching the line BH in the point B. Q.E.F.

Case 2. Suppose the four points B, C, D, P, given, being situated with¬
out the tangent HI. Join each two by the lines BD, CP meeting in G,
and cutting the tangent in H and I. Cut the tangent in A in such manner

140

THE MATHEMATICAL PRINCIPLES

[Book 1

that HA may be to IA as the rectangle un¬
der a mean proportional between CG and
GP, and a mean proportional between BH
and HD is to a rectangle under a mean pro¬
portional between GD and GB, and a mean
proportional betweeen PI and IC, and A will
be the point of contact. For if HX, a par¬
allel to the right line PI, cuts the trajectory
in any points X and Y, the point A (by the
properties of the conic sections) will come to be so placed, that HA 2 will
become to AI 2 in a ratio that is compounded out of the ratio of the rec¬
tangle XHY to the rectangle BHD, or of the rectangle CGP to the rec¬
tangle DGB; and the ratio of the rectangle BHD to the rectangle PIC.
But after the point of contact A is found, the trajectory will be described as
in the first Case. Q.E.F. But the point A may be taken either between
or without the points H and I, upon which account a twofold trajectory
may be described.

PROPOSITION XXIV. PROBLEM XVI.

To descr ibe a trajectory that shall pass through three given points, and
touch two right lines given by position.

Suppose HI, KL to be the given tangents
and B, C, D, the given points. Through any
two of those points, as B, D, draw the indefi¬
nite right line BD meeting the tangents in
the points H, K. Then likewise through
any other two of these points, as C, D, draw
the indefinite right line CD meeting the tan¬
gents in the points I, L. Cut the lines drawn
in R and S, so that HR may be to KR as
the mean proportional between BH and HD is to the mean proportional
between BK and KD; and IS to LS as the mean pioportional between
Cl and ID is to the mean proportional between CL and LD. But you
may cut, at pleasure, either within or between the points K and H, I and
L, or without them; then draw RS cutting the tangents in A and P, and
A and P will be the points of contact. For if A and P are supposed to
be the points of contact, situated anywhere else in the tangents, and through
any of the points H, I, K, L, as I, situated in either tangent HI, a right
line IY is drawn parallel to the other tangent KL, and meeting the curve
in X and Y, and in that right line there be taken IZ equal to a mean pro¬
portional between IX and IY, the rectangle XIY or IZ 2 , will (by the pro¬
perties of the conic sections) be to LP 2 as the rectangle CID is to the rect¬
angle CLD, that is (by the construction), as SI is to SL 2 , and therefore

Sec. V.]

OF NATURAL PHILOSOPHY.

141

IZ i3 to LP as SI to SL. Wherefore the points S, P, Z. are in one right
line. Moreover, since the tangents meet in G, the rectangle X1Y or IZ 2
will (by the properties of the conic sections) be to IA 2 as GP 2 is to GA 2 ,
and consequently IZ will be to IA as GP to GA. Wherefore the points
P, Z, A, lie in one right line, and therefore the points S, P, and A are in
one right line. And the same argument will prove that the points R, P,
and A are in one right line. Wherefore the points of contact A and P lie
in the right line RS. But after these points are found, the trajectory may
be described, as in the first Case of the preceding Problem. Q,.E.F.

In this Proposition, and Case 2 of the foregoing, the constructions are
the same, whether the right line XY cut the trajectory in X and Y, or
not; neither do they depend upon that section. But the constructions
being demonstrated where that right line does cut the trajectory, the con¬
structions where it does not are also known; and therefore, for brevity’s
sake, I omit any farther demonstration of them.

LEMMA XXII.

To transform figures into other figures of the same kind.

Suppose that any figure HGI is to be
transformed. Draw, at pleasure, two par¬
allel lines AO, BL, cutting any third line
AB, given by position, in A and B, and from
any point G of the figure, draw out any
right line GD, parallel to OA, till it meet
the right line AB. Then from any given
point O in the line OA, draw to the point
D the right line OD, meeting BL in d ; and
from the point of concourse raise the right
line dg containing any given angle with the right line BL, and having
such ratio to O d as DG has to OD; and g will be the point in the new
figure hgi, corresponding to the point G. And in like manner the several
points of the first figure will give as many correspondent points of the new
figure. If we therefore conceive the point G to be carried along by a con¬
tinual motion through all the points of the first figure, the point g will
be likewise carried along by a continual motion through all the points of
the new figure, and describe the same. For distinction’s sake, let us call
DG the first ordinate, dg the new ordinate, AD the first abscissa, ad the
new abscissa; O the pole, OD the abscinding radius, OA the first ordinate
radius, and O a (by which the parallelogram OABa is completed) the new
ordinate radius.

I say, then, that if the point G is placed in a right line given by posi¬
tion. the point g will be also placed in a right line given by position. If
the point G is placed in a conic section, the point g will be likewise placed

142

THE MATHEMATICAL PRINCIPLES

[Book 1.

in a conic section. And here I understand the circle as one of the conic
sections. But farther, if the point G is placed in a line of the third ana¬
lytical order, the point g will also be placed in a line of the third order,
and so on in curve lines of higher orders. The two lines in which the
points G, g, are placed, will be always of the same analytical order. For
as ad is to OA, so are O d to OD, dg to DG, and AB to AD; and there¬
fore AD is equal to — ^ , and DG equal Now if the

point G is placed in a right line, and therefore, in any equation by which
the relation between the abscissa AD and the ordinate GD is expressed,
those indetermined lines AD and DG rise no higher than to one dimen¬

sion, by writing this equation

OA X AB

in place of AD, and

OA X d<

ad r . ad

in place of DG, a new equation will be produced, in which the new ab¬
scissa ad and new ordinate dg rise only to one dimension; and which
therefore must denote a right line. But if AD and DG (or either of
them) had risen to two dimensions in the first equation, ad and dg would
likewise have risen to two dimensions in the second equation. And so on
in three or more dimensions. The indetermined lines, ad, dg in the
second equation, and AD, DG, in the first, will always rise to the same
number of dimensions; and therefore the lines in which the points G, g ,
are placed are of the same analytical order.

I say farther, that if any right line touches the curve line in the first
figure, the same right line transferred the same way with the curve into
the new figure will touch that curve line in the new figure, and vice versa.
For if any two points of the curve in the first figure are supposed to ap¬
proach one the other till they come to coincide, the same points transferred
will approach one the other till they come to coincide in the new figure;
and therefore the right lines with which those points are joined will be¬
come together tangents of the curves in both figures. I might have given

demonstrations of these assertions in a more geometrical form; but I study
to be brief.

Wherefore if one rectilinear figure is to be transformed into another, we
need only transfer the intersections of the right lines of which the first
figure consists, and through the transferred intersections to draw right lines
in the new figure. But if a curvilinear figure is to be transformed, we
must transfer the points, the tangents, and other right lines, by means of
which the curve line is defined. This Lemma is of use in the solution of
the more difficult Problems; for thereby we msgr transform the proposed
figures, if they are intricate, into others that are more simple. Thus any
right lines converging to a point are transformed into parallels, by taking
for the first ordinate radius any right line that passes through the point
of concourse of the converging lines, and that because their point of con-

Sec. V.l

OF NATURAL PHILOSOPHY.

143

course is by this means made to go off in infinitum ; and parallel lines
are such as tend to a point infinitely remote. And after the problem is
solved in the new figure, if by the inverse operations we transform the
new into the first figure, we shall have the solution required.

This Lemma is also of use in the solution of solid problems. For as
often as two conic sections occur, by the intersection of which a problem
may be solved, any one of them may be transformed, if it is an hyperbola
or a parabola, into an ellipsis, and then this ellipsis may be easily changed
into a circle. So also a right line and a conic section, in the construc¬
tion of plane problems, may be transformed into a right line and a circle

PROPOSITION XXV. PROBLEM XVII.

To describe a trajectory that shall pass through two given points , and

touch three right lines given by position.

Through the concourse of any two of the tangents one with the other,
and the concourse of the third tangent with the right line which passes
through the two given points, draw an indefinite right line; and, taking
this line for the first ordinate radius, transform the figure by the preceding
Lemma into a new figure. In this figure those two tangents will become
parallel to each other, and the third tangent will be parallel to the right
line that passes through the two given points. Suppose hi, kl to be those
two parallel tangents, ik the third tangent, and hi a right line parallel
thereto, passing through those points a, b,
through which the conic section ought to pass
in this new figure; and completing the paral-
lelogra n hikl, let the right lines hi, ik, kl be
so cut in c, d, e, that he may be to the square
root of the rectangle ahb, ic, to id, and ke to
kd, as the sum of the right lines hi and kl is
to the sum of the three lines, the first whereof ’
is the right line ik, and the other two are the
square roots of the rectangles ahb and alb ; and c, d, e , will be the points
of contact. For by the properties of the conic sections, he 2 to the rectan¬
gle ahb, and ic 2 to id 2 , and ke 2 to kd 2 , and el 2 to the rectangle alb, are all
in the same ratio; and therefore he to the square root of ahb, ic to id, ke
to kd, and el to the square root of alb, are in the subduplicate of that
ratio; and by composition, in the given ratio of the sum of all the ante¬
cedents hi + kl, to the sum of all the consequents ahb 4 ik - alb .
Wherefore from that given ratio we have the points of contact c, d, e, in
the new figure. By the inverted operations of the last Lemma, let those
points be transferred into the first figure, and the trajectory will be there
described by Prob. XIV. Q.E.F. But according as the points a, b, fall
between the points //, l, or without them, the points c, d, e, must be taken

144

THE MATHEMATICAL PRINCIPLES

Book Lj

either between the points, A, i, k, l, or without them. If one of the points
a, b, falls between the points A, i, and the other ivithout the points A, A
the Problem is impossible.

PROPOSITION XXVI. PROBLEM XYIII.

To describe a trajectory that shall pass through a given point , and touch
four right lines given by position.

From the common intersections, of any two
of the tangents to the common intersection of
the other two, draw an indefinite right line; and
taking this line for the first ordinate radius,
transform the figure (by Lem. XXII) into a new
figure, and the two pairs of tangents, each of
which before concurred in the first ordinate ra¬
dius, will now become parallel. Let hi and kl, h
ik and III , be those pairs of parallels completing the parallelogram hikl .
And let p be the point in this new figure corresponding to the given point
in the first figure. Through O the centre of the figure draw pq\ and Oq
being equal to Op , q will be the other point through which the conic sec¬
tion must pass in this new figure. Let this point be transferred, by the
inverse operation of Lem. XXII into the first figure, and there we shall
have the two points through which the trajectory is to be described. But
through those points that trajectory may be described by Prop. XVII.

LEMMA XXIII.

If two right lines , as AC, BD given by position , and terminating in
given points A, B, are in a given ratio one to the other , and the right
line CD, by which the indetermined points C, D are joined is cut in
K in a given ratio ; I say, that the point K will be placed in a right
line given by position.

For let the right lines AC, BD meet in
E, and in BE take BG to AE as BD is to
AC, and let FD be always equal to the given
line EG; and, by construction, EC will be
to GD, that is, to EF, as AC to BD, and
therefore in a given ratio ; and therefore the
triangle EFC will be given in kind. Let
CF be cut in L so as CL may be to CF in the ratio of CK to CD; and
because that is a given ratio, the triangle EFL will be given in kind, and
therefore the point L will be placed in the right line EL given by position.
Join LK, and the triangles CLK, CFI) will be similar; and because FD
is a given line, and LK is to FD in a given ratio, LK will be also given

L -

.V

A

\

E I£ fjr T5

OF NATURAL PHILOSOPHY.

145

Sec. V.]

To this let EH be taken equal, and ELKH will be always a parallelogram.
And therefore the point K is always placed in the side HK (given by po
sition) of that parallelogram. Q.E.D.

Cor. Because the figure EFLC i3 given in kind, the three right lines
EF, EL, and EC, that is, GD, HK, and EC, will have given ratios to
each other.

LEMMA XXIY.

If three right, lines, two whereof are parallel, and given by position, touch
any conic section ; I say, that the semi-diameter of the section whi Ji
is parallel to those two is a mean proportional between the segments
of those two that are intercepted between the points of contact a nd the
third tangent .

Let AF, GB be the two parallels touch¬
ing the conic section ADB in A and B ;

EF the third right line touching the conic
section in I, and meeting the two former
tangents in F and G, and let CD be the
semi-diameter of the figure parallel to
those tangents; I say, that AF, CD, BG
are continually proportional.

For if the conjugate diameters AB, DM
meet the tangent FG in E and H, and cut one the other in C, and the
parallelogram IKCL be completed ; from the nature of the conic sections,
EC will be to CA as CA to CL ; and so by division, EC — CA to CA
CL, orEAto AL; and by composition, EA to EA + AL or EL, as EC to
EC-fCA or EB; and therefore (because of the similitude of the triangles
EAF, ELI, ECH, EBG) AF is to LI as CH to BG. Likewise, from thi
nature of the conic sections, LI (or CK) is to CD as CD to CH; and
therefore [ex reqno pertnrhate) AF is to CD as CD to BG. Q.E.D.

Cor. 1. Hence if two tangents FG, PQ, meet two parallel tangents AF,
BG in F and G, P and Q,, and cut one the other in O; AF {ex cequo per-
turbot t ) will be to BQ, as AP to BG, and by division, as FP to GQ, and
therefore as FO to OG.

Cor. 2. Whence also the two right lines PG, FQ, drawn through the
points P and G, F and Q,, will meet in the right line ACB passing through
the centre of the figure and the points of contact A, B.

LEMMA XXY.

Ff four sides of a parallelogram indefinitely produced touch any conic
section, and are cut by a fifth tangent ; I say, that, taking those seg¬
ments of any tico conterminous sides that terminate in opposite angles

10

146

THE MATHEMATICAL PRINCIPLES

[Book 1.

of the parallelogram, either segment is to the side from which it is
cut off as that part of the other conterminous side which is intercepted
between the point of contact and the third side is to the other segment,
Let the four sides ML, IK, KL, MI,
of the parallelogram MLIK touch the F
conic section in A, B, C, D ; and let the
fifth tangent FQ cut those sides in F,

(4, H, and E ; and taking the segments
ME, KQ of the sides MI, KJ, or the
segments KH, MF of the sides KL,

ML, 1 say, that ME is to MI as BK to
KQ; and KH to KL as AM to MF.

For, by Cor. 1 of the preceding Lemma, ME i3 to El as (AM or) BK to
BQ; and, by composition, ME is to MI as BK to KQ. Q.E.D. Also
KH is to HL as (BK or) AM to AF; and by division, KH to KL as AM
to MF. Q.E.D.

Cor. 1. Hence if a parallelogram IKLM described about a given conic
section is given, the rectangle KQ X ME, as also the rectangle KH X MF
equal thereto, will be given. For, by reason of the similar triangles KQH
MFE, those rectangles are equal.

Cor. 2. And if a sixth tangent eq is drawn meeting the tangents Kl.
MI in q and e, the rectangle KQ X ME will he equal to the rectangle
K^ X Me, and KQ will be to Me as Kq to ME, and by division as
Qq to Ee.

Cor. 3. Hence, also, if E q, eQ, are joined and bisected, and a right line
is drawn through the points of bisection, this right line will pass through
the centre of the conic section. For since Q q is to Ee as KQ to Me, the
same right line will pass through the middle of all the lines E^, eQ, MK
(by Lem. XXIII), and the middle point of the right line MK is the
centre of the section.

PROPOSITION XXVII. PROBLEM XIX.

To describe a trajectory that may touch five right lines given by position.

Supposing ABG, BCF,

GCD, FDE, EA to be the
tangents given by position.

Bisect in M and N, AF, BE,
the diagonals of the quadri¬
lateral figure ABFE con¬
tained under any four of
them; and (by Cor. 3, Lem.

XXV) the right line MN
drawn through the points of

Sec. V.l

OF NATURAL PHILOSOPHY.

147

bisection will pass through the centre of the trajectory. Again, bisect in
P and Q, the diagonals (if I may so call them) Bl), GF of the quadrila¬
teral figure BGI)F contained under any other four tangents, and the right
line PQ, drawn through the points of bisection will pass through the cen¬
tre of the trajectory; and therefore the centre will be given in the con¬
course of the bisecting lines. Suppose it to be O. Parallel to any tan¬
gent BC draw KL at such distance that the centre 0 may be placed in the
middle between the parallels; this KL will touch the trajectory to be de
scribed. Let this cut any other turn tangents GOD, FJ)E, in L and K.
Through the points C and K, F and L, where the tangents not parallel,
CL, FK meet the parallel tangents OF, KL, draw CK, FL meeting in
11; and the right line OR drawn and produced, will cut the parallel tan¬
gents CF, KL, in the points of contact. This appears from Cor. 2, Lem.
XXIV. And by the same method the other points of contact may be
found, and then the trajectory may be described by Prob. XIV. Q.E.F.

SCHOLIUM.

Under the preceding Propositions are comprehended those Problems
wherein either the centres or asymptotes of the trajectories are given. For
when points and tangents and the centre are given, as many other points
and as many other tangents are given at an equal distance on the other
side of the centre. And an asymptote is to be considered as a tangent, and
its infinitely remote extremity (if we may say so) is a point of contact.
Conceive the point of contact of any tangent removed in infinitum , and
the tangent will degenerate into an asymptote, and the constructions of
the preceding Problems will be changed into the constructions of those
Problems wTerein the asymptote is given.

After the trajectory is described, we may
find its axes and foci in this manrnr. In the
construction and figure of Lem. XXI, let those
legs BP, CP, of the moveable angles PBN,

PCN, by the concourse of which the trajec¬
tory was described, be made parallel one to
the other; and retaining that position, let
them revolve about their poles 1 *, C, in that
figure. In the mean while let the other legs
CN, BN, of those angles, by their concourse
K or k, describe the circle BKGC. Let O be the centre of this circle;
and from this centre upon the ruler MN, wherein those legs CN, BN did
concur while the trajectory was described, let fall the perpendicular OH
meeting the circle in K and L. And when those other legs CK, BK meet
in the point K that is nearest to the ruler, the first legs CP, BP will be
pa-allel to the greater axis, and perpendicular on the lesser; and the con-

148

THE MATHEMATICAL PRINCIPLES

[Book L

trary will happen if those legs meet in the remotest point L. Whence il
the centre of the trajectory is given, the axes will be given ; and those be¬
ing given, the foci will, be readily found.

CK, BK, when the first legs CP, BP meet in the fourth given point, will
be the ruler MN, by means of whicli the trajectory may be described
Whence also on the other hand a trapezium given in kind (excepting a
few cases that are impossible) may be inscribed in a given conic section.

There are also other Lemmas, by the help of which trajectories given m
kind may be described through given points, and touching given lines.
Of such a sort is this, that if a right line is drawn through any point
given by position, that may cut a given conic section in two points, and
me distance of the intersections is bisected, the point of bisection will
to ich ano her conic section of the same kind with the former, and having

' o

its axes parallel to the axes of the former. But I hasten, to things of
greater use.

LEMMA XXVI.

To place the three angles of a triangle , given both in kind and magni¬
tude, in respect of as many right lines given by position, provided they
are not all parallel among themselves , in such manner that the s e^eral
angles may touch the several lines.

Three indefinite right lines AB, AC, BC, are
given by position, and it is required so to place
the triangle DEF that its angle D may touch
tbe line AB, its angle E the line AC, and
its angle F the line BC. Upon DE, DF, and
FF, describe three segments of circles DRE,

DGF, EMF, capable of angles equal to the
nngles BAC, ABC, ACB respectively. But those segments are to be de¬
scribed t' wards such sides of the lines DE, DF, EF, that the letters

Snc. V.] of natural philosophy. 14*1

DRED may turn round about in the same order with the letters 13ACB;
the letters DGFD in the same order with the letters ABCA; and the
letters EMFE in the same order with the letters ACBA; then, completing
th :>se segmerts into entire circles let the two former circles cut one the
other in G, and suppose P and Q, to be their centres. Then joining GP,
PQ, take Ga to AB as GP is to PQ,; and about the centre G, with the
interval Ga, describe a circle that may cut the first circle DGE in a.
Join aD cutting the second circle DFG in b, as well as aE cutting the
third circle EMF in c. Complete the figure ABC def similar and equal
to the figure aicDEF: I say, the thing is done.

For drawing Fc meeting aD in n t
and joining aG, bG, QG, Q.D, PD, by
construction the angle EaD is equal to
the angle CAB, and the angle acF equal
to the angle ACB; and therefore the
triangle anc equiangular to the triangle
ABC. Wherefore the angle anc or FaD
is equal to the angle ABC, and conse-
< uently to the angle F&D; and there¬
fore the point n falls on the point b.

Moreover the angle GPQ, which is half
the angle GPD at the centre, is equal
to the angle GaD at the circumference
and the angle GQP, which is half the angle GQD at the centre, is equal
to the complement to two right angles of the angle G6D at the circum¬
ference, and therefore equal to the angle Gba. Upon which account the
triangles GPQ, Gab, are similar, and Ga is to ab as GP to PQ.; that is
'by construction), as Ga to AB. Wherefore ab and AB are equal; and
consequently the triangles abc , ABC, which we have now proved to be
similar, are also equal. And therefore since the angles D, E, F, of the
triangle DEF do respectively touch the sides ab, ac, be of the triangle
abc, the figure ABC def may be completed similar and equal to the figure
a&cDEF, and by completing it the Problem will be solved. Q.E.F.

Cor. Hence a right line may be drawn whose parts given in length may
be intercepted between three right lines given by position. Suppose the
triangle DEF, by the access of its point D to the side EF, and by having
the sides DE, DF placed in directum to be changed into a right line
whose given part DE is to be interposed between the right lines AB, AC
given by position; and its given part DF is to be interposed between the
right lines AB, BC, given by position; then, by applying the preceding
construction to this case, the Problem will be solved.

50

THE MATHEMATICAL PRINCIPLES

[Book 1.

PROPOSITION XXVIII. PROBLEM XX.

To describe a trajectory given both in kind and magnitude , given parts
of which shall be interposed between three right lines given by position.
Suppose a trajectory is to be described that
may be similar and equal to the curve line DEF,

♦and may be cut by three right lines AB, AC,

BC, given by position, into parts DE and EF,
similar and equal to the given parts of this
curve line.

Draw the right lines DE, EF, DF: and
place the angles D, E, F, of this triangle DEF, so
as to touch those right lines given by position (by
Lem. XXVI). Then about the triangle describe
the trajectory, similar and equal to the curve DEF.

Q.E.F.

LEMMA XXVII.

To describe a trapezium given in kind , the angles whereof may be su
placedj in respect of four right lines given by position , that are neither
all parallel among themselves , nor converge to one common point , that
the several angles may touch the several lines .

Let the four right lines ABC, AD, BD, CE, be
given by position ; the first cutting the second in A,
the third in B, and the fourth in C; and suppose a
trapezium fghi is to be described that may be similar
to the trapezium FCHI, and whose angle f equal to
the given angle F, may touch the right line ABC; and
the other angles g , h, i, equal to the other given angles,

G, H, I, may touch the other lines AD, BD, CE, re¬
spectively. Join FH, and upon FG. FH, FI describe
as many segments of circles FSG, FTH, FVI, the first
of which FSG may be capable of an angle equal to
the angle BAD; the second FTH capable of an angle
equal to the angle CBD ; and the third FVI of an angle equal to the angle
ACE. Bud the segments are to be described towards those sides of the
linfes FG, FH, FI, that the circular order of the letters FSGF may be
the same as of the letters BADB, and that the letters FTHF may turn
about in the same order as the letters CBDC and the letters FVIF in the
game order as the letters ACE A. Complete the segments into entire cir¬
cles, and let P be the centre of the first circle FSG, Q the centre of the
second FTH. Join and produce both ways the line PQ, and in it take
OR in the same ratio to PQ, as BC has to AB. But QR is to be taken
towards that side of the point Q, that the order of the letters P, Q, R

OF NATURAL PHILOSOPHY.

151

Sec. V.J

may be the same as of the letters A, B, C;
and about the centre R with the interval
RF describe a fourth circle FNc cutting
the third circle FVI in c. Join Fc cut¬
ting the first circle in a, and the second in
t . Draw rtG, 5H, cl, and let the figure
ABC fghi be made similar to the figure
</5cFGHI; and the trapezium fghi will
be that which was required to be de¬
scribed.

For let the two first circles FSG, FTH
cut one the other in K ; join PK, QK,

RK, aK, 5K, cK, and produce Q,P to L.

The angles FaK, F5K, FcK at the circumferences are the halves of the
angles FPK, FQJC, FRK, at the centres, and therefore equal to LPK,
LQK, LRK, the halves of those angles. Wherefore the figure PQRK is
iquiangular and similar to the figure abcK, and consequently ab is to be
<;s PQ, to Q,R. that is, as AB to BC. But by construction, the angles
r Ag, /lMi,fCi, are equal to the angles F«G, F5H, FcL And therefore
the figure ABC fghi may be completed similar to the figure o^cFGHI.
Which done a trapezium fghi will be constructed similar to the trapezium
FGHI, and which by its angles f g , h, i will touch the right lines ABC,
AD, BD, CE. Q.E.F.

Cor. Hence a right line may be drawn whose parts intercepted in a
given order, between four right lines given by position, shall have a given
proportion among themselves. Let the angles FGH, GHI, be so far in¬
creased that the right lines FG, GH, HI, may lie in directum ; and by
constructing the Problem in this case, a right line fghi will be drawn,
whose parts fg, gh , hi, intercepted between the four right lines given by
position, AB and AD, AD and BD, BD and CE, will be one to another
as the lines FG, GH, HI, and will observe the same order among them¬
selves. But the same thing may be more readily done in this manner.

Produce AB to K and BD to L,

so as BK may be to AB as HI to tI

GH; and DL to BD as GI to FG;
and join KL meeting the right line
CE in i. Produce iL to M, so as
LM may be to iL as GH to HI;
then draw MQ, parallel to LB, and
meeting the right line AD mg', and
join gi cutting AB, BD in f A; I
say, the thing is done.

For let Mg* cut the right line AB in Q, and AD the right line KL in

in

be,

i

.52

THE MATHEMATICAL PRINCIPLES

[Book I.

S, and draw AP parallel to BD, and meeting ih in P, and §*M to LA (g:
to hi, Mi to hi, GI to HI, AK to BK) and AP to BL, will be in the same
ratio. Cut DL in R, so as DL to RL may be in that same ratio; and be¬
cause gS to gM, AS to AP. and DS to DL are proportional; therefore
(ex cequo) as gS to L h, so will AS be to BL, and DS to RL; and mixtly,
BL — RL to LA— BL, as AS— DS to gS — AS. That is, BR is to
BA as AD is to A g, and therefore as BD to "Q,. And alternately BR is
to BD as BA to g’Q, or as fh to fg. But by construction the line BL
was cut in D and R in the same ratio as the line FI in C and H; and
therefore BR is to BD as FH to FG. Wherefore fh is to fg as FH to
FG. Since, therefore, gi to hi likewise is as Mi to Li, that is, as GI to
III, it is manifest that the lines FI, /i, are similarly cut in G and H, g
and A. Q.F.F.

In the construction of this Corollary, after the line LK is drawn cutting
CE in i, we may produce iE to Y, so as EY may be to Ei as FH to HI,
arid then draw Yf parallel to BD. It will come to the same, if about the
centre i with an interval IH, we describe a circle cutting BD in X, and
produce iX to Y so as iY may be equal to IF, and then draw Yf parallel
to BD.

Sir Christopher Wren and Dr. Wallis have long ago given other solu¬
tions of this Problem.

PROPOSITION XXIX. PROBLEM XXL
To describe a trajectory given in kind, that may be cut by four right
lines given by position, into parts given in order , kind, and proportion.
Suppose a trajectory is to be described that may be
similar to the curve line FGHI, and whose parts,
similar and proportional to the parts FG, GH, HI of
the other, may be intercepted between the right lines
AB and AD, AD, and BD, BD and CE given by po¬
sition, viz., the first between the first pair of those lines,
the second between the second, and the third between
the third. Draw the right lines FG, GH, HI, FI;
and (by Lem. XXYII) describe a trapezium fghi that
may be similar to the trapezium FGHI, and whose an¬
gles/, g, A, i, may touch the right lines given by posi¬
tion AB, AD, BD, CE, severally according to their order. And then about
this trapezium describe a trajectory, that trajectory will be similar to the
curve line FGHI.

SCHOLIUM.

This problem may be likewise constructed in the following manner.
Joining FG, GH, HI, FI, produce GF to Y, and join FH, IG, and make

Sec. VI]

OF NATURAL PHILOSOPHY.

153

the angles CAK. DAL equal to
the angles FGH, VFH. Let
AK, AL meet the right line
BD in K and L, and thence
draw KM, LN, of which let
KM make the angle A KM equal
to the angle CHI, and be itself
to AK as HI is to GH; and let

LN make the angle ALN equal to the angle FHI, and be itself
to AL as HI to FH. But AK, KM. AL, LN are to be drawn
towards those sides of the lines AD, AK, AL, that the letters
OAKMC, ALKA, DALND may be carried round in the same
order as the letters FGHIF; and draw MN meeting the right,
line CE in i. Make the angle IEP equal to the angle IGF,
and let PE be to Ei as FG to GI; and through P draw PQ/' that may
with the right line ADE contain an angle PQ.E equal to the angle FIG,
and may meet the right line AB in fi and join fi. But PE and PQ are
to he drawn towards those sides of the lines CE, PE, that the circular
order of the letters PEiP and PEQ,P may be the same as of the letters
FGHIF; and if upon the line fi , in the same order of letters, and similar
to the trapezium FGHI, a trapezium fghi is constructed, and a trajectory
given in kind is circumscribed about it, the Problem will be solved.

So far concerning the finding of the orbits. It remains that we deter¬
mine the motions of bodies in the orbits so found.

SECTION VI.

How the fjiotioas are to be found in given orbits.

PROPOSITION XXX. PROBLEM XXII.

To find at any assigned time the place of a body moving in a given

parabolic trajectory.

Let S be the focus, and A the principal vertex of
the parabola; and suppose 4AS X M equal to the
parabolic area to be cut off APS, which either was
described by the radius SP, since the body's departure
from the vertex, or is to be described thereby before
its arrival there. Now the quantity of that area to
be cut off is known from the time which is propor¬
tional to it. Bisect AS in G, and erect the perpendicular GH equal to
3M, and a circle described about th 3 centre H, with the interval HS, will
cut the parabola in the place P required. For letting fall PO perpendic¬
ular on the axis, and drawing PH, there will be AG 2 -f- GII 2 (—IIP 2

AO —AGl 2 + PO — GH| 2 ) = AO 2 + PO 2 —2CA ) —?GTI f PO %

154

THE MATHEMATICAL PRINCIPLES

[Book I

AG* + GH 2 . Whence 2GH X PO (=*= AO 2 -f PO 2 — 2GAO) == AO*

PO 2

-f | PO 2 . For AO 2 write AO X'^jg; then dividing all the terms by
3PO ; and multiplying them by 2AS, we shall have |GH X AS (= UO
X PO + iAS X PO_“±“?x PO_i^“ X Pol*

the area APO — SPO)| = to the area APS. But GH was 3M, and
therefore 5 GH X AS is 4AS X M. Wherefore the area cut off APS is
equal to the area that was to be cut off 4AS X M. Q.E.D.

Cor. 1. Hence GH is to AS as the time in which the body described
the arc AP to the time in which the body described the arc between the
vertex A and the perpendicular erected from the focus S upon the axis.

Cor. 2. And supposing a circle ASP perpetually to pass through the
moving body P, the yelocity of the point H is to the velocity which the
body had in the vertex A as 3 to 8 ; and therefore in the same ratio is
the line GH to the right line which the body, in the time of its moving
from A to P, would describe with that velocity which it had in the ver¬
tex A.

Cor. 3. Hence, also, on the other hand, the time may be found in which
the body has described any assigned arc AP. Join AP, and on its middle
point erect a perpendicular meeting the right line GH in H.

LEMMA XXVIII.

There is no oval figure whose area , cut off by right lines at pleasure, can.
be universally found by means of equations of any number of finite
terms and dimensions.

Suppose that within the oval any point is given, about which as a pole
a right line is perpetually revolving with an uniform motion, while in
that right line a moveable point going out from the pole moves always
forward with a velocity proportional to the square of that right line with¬
in the oval. By this motion that point will describe a spiral with infinite
circumgyrations. Now if a portion of the area of the oval cut off by that
right line could be found by a finite equation, the distance of the point
from the pole, which is proportional to this area, might be found by the
same equation, and therefore all the points of the spiral might be found
by a finite equation also; and therefore the intersection of a right line
given in position with the spiral might also be found by a finite equation.
But every right line infinitely produced cuts a spiral in an infinite num¬
ber of points ; and the equation by which any one intersection of two lines
is found at the same time exhibits all their intersections by as many roots,
and therefore rises to as many dimensions as there are intersections. Bo-
cause two circles mutually cut one another in two points, one of those in*

Sfc. VI.] of natural philosophy. 155

terscctions is not to be found but by an equation of two dimensions, bv
which the other intersection may be also found. Because there may be
four intersections of two conic sections, any one of them is not to be found
universally, but by an equation of four dimensions, by which they may be
all found together. For if those intersections are severally sought, be¬
cause the law and condition of all is the same, the calculus will be the
same in every case, and therefore the conclusion always the same, which
must therefore.comprehend all those intersections at once within itself, and
exhibit them all indifferently. Hence it is that the intersections of the
conic sp^ions with the curves of the third order, because they may amount
to six, (x,me out together by equations of six dimensions; and the inter¬
sections of two curves of the third order, because they may amount to nine,
come out together by equations of nine dimensions. If this did not ne¬
cessarily happen, we might reduce all solid to plane Problems, and those
higher than solid to solid Problems. But here I speak of curves irreduci¬
ble in power. For if the equation by which the curve is defined may be
reduced to a lower power, the curve will not be one single curve, but com¬
posed of two, or more, whose intersections may be severally found by different
calculusses. After the same manner the two intersections of right lines
with the conic sections come out always by equations of two dimensions; the
three intersections of right lines with the irreducible curves of the third
order by equations of three dimensions; the four intersections of right
lines with the irreducible curves of the fourth order, by equations of four
dimensions; and so on in infinitum. Wherefore the innumerable inter¬
sections of a right line with a spiral, since this is but one simple curve
and not reducible to more curves, require equations infinite in r. amber of
dimensions and roots, by which they may be all exhibited together. For
the law and calculus of all is the same. For if a perpendicular is let fall
from the pole upon that intersecting right line, and that perpendicular
together with the intersecting line revolves about the pole, the intersec¬
tions of the spiral will mutually pass the one into the other; and that
which was first or nearest, after one revolution, will be the second; after
two, the third; and so on: nor will the equation in the mean time be
changed but as the magnitudes of those quantities are changed, by which
the position of the intersecting line is determined. Wherefore since those
quantities after every revolution return to their first magnitudes, the equa¬
tion will return to its first form; and consequently one and the same
equation will exhibit all the intersections, and will therefore have an infi¬
nite number of roots, by which they may be all exhibited. And therefore
the intersection of a right line with a spiral cannot be universally found by
any finite equation; and of consequence there is no oval figure whose area*
cut off by right lines at pleasure, can be universally exhibited by anj
such equation.

THE MATHEMATICAL PRINCIPLES

156

[Book 1

By the same argument, if the interval of the pole and point by which
the spiral is described is taken proportional to that part of the perimeter
of the oval which is cut off, it may be proved that the length of the peri¬
meter cannot be universally exhibited by any finite equation. But here I
speak of ovals that are not touched by conjugate figures running out in
infinitum.

Cor. Hence the area of an ellipsis, described by a radius drawn from
the focus to the moving body, is not to be found from the time given by a
finite equation ; and therefore cannot be determined by the description ol
curves geometrically rational. Those curves I call geometrically rational,
all the points whereof may be determined by lengths that are definable
by equations; that is, by the complicated ratios of lengths. Other cur ves
(such as spirals, quadratrixes, and cycloids) I call geometrically irrational.
For the lengths which are or are not as number to number (according to
the tenth Book of Elements) are arithmetically rational or irrational.
And therefore I cut off an area of an ellipsis proportional to the time in
which it is described by a curve geometrically irrational, in the following
manner.

PROPOSITION XXXI. PROBLEM XXIII.

To find the place of a body moving in a given elliptic trajectory at any

assigned time .

Suppose A to be
the principal vertex,

S the focus, and O
the centre of the
ellipsis APB; and
let P be the place of
the body to be found.

Produce OA to G so
as OG may be to OA
as OA to OS. Erect
the perpendicular GH; and about the centre O, with the interval OG, de¬
scribe the circle GEF ; and on the ruler GH, as a base, suppose the wheel
GEF to move forwards, revolving about its axis, and in the mean time by
its point A describing the cycloid ALI. Which done, take GK to the
perimeter GEFG of the wheel, in the ratio of the time in which the body
proceeding from A described the arc AP, to the time of a whole revolution
in the ellipsis. Erect the perpendicular KL meeting the cycloid in L ;
then LP drawn parallel to KG will meet the ellipsis in P, the required
place of the body.

For about the centre O with the interval OA describe the semi-circle
AQB, and let LP, produced, if need be, meet the arc AQ, in Q, and join

Sec. VLj

OF NATURAL PHILOSOPHY.

157

SQ, OQ. Let OQ meet the arc EFG in F, and upon OQ kt fall the
perpendicular Sll. The area APS is as the area AQS, that is, as the
diiference between the sector OQA and the triangle OQS, or as the difLr-
ence of the rectangles pQ X AQ, and : ’OQ X SR, that is, because
is given, as the difference between the arc AQ, and the right line Sll; and
therefore (because of the equality of the given ratios SR to the sine of the
arc AQ, OS to OA, OA to OG, AQ to GF; and by division, AQ— SR
to Qp — s ine of the arc AQ) as GK, the difference between the arc G1
and the sine of the arc AQ. Q.E.D.

SCHOLIUM.

But since the description of this curve ng — ^

is difficult, a solution by approximation
will be preferable. First, then, let there \

be found a certain angle B which may // \\ \\

be to an angle of 57,29578 degrees, / / \\ \

which an arc equal to the radius subtends, j r \

as SH, the distance of the foci, to AB, a s r> o ± b
the diameter of the ellipsis. Secondly, a certain length L, which may be to
the radius in the same ratio inversely. And these being found, the Problem
may be solved by the following analysis. By any construction (or even
by conjecture), suppose we know P the place of the body near its true
place p. Then letting fall on the axis of the ellipsis the ordinate PR
from the proportion of the diameters of the ellipsis, the ordinate RQ ol
the circumscribed circle AQB will be given ; which ordinate is the sine of
the angle AOQ, supposing AO to be the radius, and also cuts the ellipsis
in P. It will .be sufficient if that angle is found by a rude calculus in
numbers near the truth. Suppose we also know the angle proportional to
the time, that is, which is to four right aigles as the time in which the
body described the arc A p, to the time of one revolution in the ellipsis.
Let this angle be N. Then take an angle D, which may be to the angle
B as the sine of the angle AOQ to the radius; and an angle E which
may be to the angle N — AOQ +D as the length L to the same length
L diminished by the cosine of the angle AOQ, when that angle is less
than a right angle, or increased thereby when greater. In the next
place, take an angle F that may be to the angle B as the sine of the angle
10Q + E to the radius, and an angle G, that may be to the angle N —
AOQ — E + F as the length L to the same length L diminished by the
cosine of the angle AOQ T E, when that angle i3 less than a right angle,
or increased thereby when greater. For the third time take an angle H,
that may be to the angle B as the sine of the angle AOQ r E + G to the
radius; and an angle I to the angle N — AOQ — E — G -f H, as the

58

THE MATHEMATICAL PRINCIPLES

|B(Ok L

length L is to the same length L diminished by the cosine of the angle
AOQ -f- E + G, when that angle is less than a right angle, or increased
thereby when greater. And so we may proceed in infinitum. Lastly,
take the angle AOq equal to the angle AOQ, -f- E + G +1 -{-, &c. and
from its cosine Or and the ordinate pr, which is to its sine qr as the lesser
axis of the ellipsis to the greater, w e shall have p the correct place of the
body. When the angle N — AOQ + D happens to be negative, the
sign + of the angle E must be every where changed into —, and the sign —
into +. And the same thing is to be understood of the signs of the angles
G and I, when the angles N — AOQ — E + P, and N — AOQ — E —
G + H come out negative. But the infinite series AOQ + E -f- G -j- I +,
&c. converges so very fast, that it will be scarcely ever needful to pro¬
ceed beyond the second term E. And the calculus is founded upon
this Theorem, that the area APS is as the difference between the arc
AQ and the right line let fall from the focus S perpendicularly upon the
radius OQ.

And by a calculus not unlike, the Problem
is solved in the hyperbola. Let its centre be
O, its vertex A, its focus S, and asymptote
OK; and suppose the quantity of the area to
be cut off is known, as being proportional to
the time. Let that be A, and by conjecture
suppose we know the position of a right i ne
SP, that cuts off an area APS near the truth.

Join OP, and from A and P to the asymptote ° T -A. S

draw AI, PK parallel to the other asymptote; and by the table of loga¬
rithms the area AIKP will be given, and equal thereto the area OPA,
which subducted from the triangle OPS, will leave the area cut off APS.
And by applying 2APS — 2A, or 2A — 2A PS, the double difference of
the area A that was to be cut off, and the area APS that is cut off, to the
line SN that is let fall from the focus S, perpendicular upon the tangent
TP, we shall have the length of the chord PQ. Which chord PQ is to
be inscribed between A and P, if the area APS that is cut off be greater
than the area A that was to be cut off, but towards the contrary side of the
point P, if otherwise: and the point Q will be the place of the body more
accurately. And by repeating the computation the place may be found
perpetually to greater and greater accuracy.

And by such computations we have a general
analytical resolution of the Problem. But the par¬
ticular calculus that follows is better fitted for as¬
tronomical purposes. Supposing AO, OB, OD, to
be the semi-axis of the ellipsis, and L its latus rec¬
tum, and D the difference betwixt the lesser semi-

OF NATURAL PHILOSOPHY.

159

Sec. VII.]

axis OD, and JL the half of the latus rectum : let an angle Y be found, whose
sine may be to the radius as the rectangle under that difference D, and
AO -f- OD the half sum of the axes to the square of the greater axis AB.
Find also an angle Z, whose sine may be to the radius as the double rec¬
tangle under the distance of the foci SH and that difference D to triple
the square of half the greater semi-axis AO. Those angles being once
found, the place of the body may be thus determined. Take the angle T
proportional to the time in which the arc BP was described, or equal to
what is called the mean motion; and an angle V the first equation of the
mean motion to the angle Y, the greatest first equation, as the sine of
double the angle T is to the radius ; and an angle X, the second equation,
to the angle Z, the second greatest equation, as the cube of the sine of the
angle T is to the cube of the radius. Then take the angle BHP the mean
motion equated equal to T + X + V, the sum of the angles T, V, X,
if the angle T is less than a right angle; or equal to T + X — Y, the
difference of the same, if that angle T is greater than one and less than
two right angles; and if HP meets the ellipsis in P, draw SP, and it will
cut off the area BSP nearly proportional to the time.

This practice seems to be expeditious enough, because the angles V and
X, taken in second minutes, if you please, being very small, it will be suf¬
ficient to find two or three of their first figures. But it is likewise
sufficiently accurate to answer to the theory of the planet’s motions.
For even in the orbit of Mars, where the greatest equation of the centre
amounts to ten degrees, the error will scarcely exceed one second. But
when the angle of the mean motion equated BHP is found, the angle of
the true motion BSP, and the distance SP, are readily had by the known
methods.

And so far concerning the motion of bodies in curve lines. But it may
also come to pass that a moving body shall ascend or descend in a right
line: and I shall now go on to explain what belongs to such kind of
motions.

SECTION m

Concerning the rectilinear ascent and descent of bodies.

PROPOSITION XXXII. PROBLEM XXIV.

Supposing that the centripetal force is reciprocally proportional to the.
square of the distance of the places from the centre; it is required
to define the spaces which a body, falling directly , describes in given
times.

Case 1. If the body does not fall perpendicularly, it will (by Cor. I

160

THE MATHEMATICAL PRINCIPLES

[Book I

Prop. XIII) describe some conic section whose focus is
placed in the centre of force. Suppose that conic sec¬
tion to be A RPB and its focus S. And, first, if the
figure be an ellipsis, upon the greater axis thereof AB
describe the semi-circle ADB, and let the right line
DPC pass through the falling body, making right angles
with the axis; and drawing BS, PS, the area ASD will
be proportional to the area ASP, and therefore also to
the time. The axis AB still remaining the same, let the
breadth of the ellipsis be perpetually diminished, and
the area ASD will always remain proportional to the
time. Suppose that breadth to be diminished in infinitum ; and the orbit
APB in that case coinciding with the axis AB, and the focus S with the
extreme point of the axis B, the body will descend in the right line AC’,
and the area ABD will become proportional to the time. Wherefore the
space AC will be given which the body describes in a given time by its
perpendicular fall from the place A, if the area ABD is taken proportional
to the time, and from the point D the right line DC is let fall perpendic¬
ularly on the right line AB. Q,.E.I.

Case 2. If the figure RPB is an hyperbola, on the
same principal diameter AB describe the rectangular
hyperbola BED ; and because the areas CSP, CB/P,

SP/B, are severally to the several areas CSD, CBED,

SDEB, in the given ratio of the heights CP, CD, and
the area SPyB is proportional to the time in which
the body P will move through the arc iyB, the area
SDEB will be also proportional to that time. Let
the latus rectum of the hyperbola RPB be diminished
in infinitum, the latus transversum remaining the
same; and the arc PB will come to coincide with the
right line CB, and the focus S, wfith the vertex B,
and the right line SD with the right line BD. And therefore the area
BDEB will be proportional to the time in which the body C, by its per¬
pendicular descent, describes the line CB. Q.E.I.

Case 3. And by the like argument, if the figure
RPB is a parabola, and to the same principal ver¬
tex B another parabola BED is described, that
may always remain given while the former para¬
bola in whose perimeter the body P moves, by
having its latus rectum diminished and reduced
to nothing, comes to coincide with the line CB,
the parabolic segment BDEB will be proportional
to the time in which that body P or C will descend to the centre S or
Q.E.T

B

Skc. V/I.J

OF NATURAL PHILOSOPHY.

PROPOSITION XXXIII. THEOREM IX.

77/e things above found being supposed . / say, /A/// ike velocity of a Jai¬
ling body in any place C is to the velocity of a body, describing a
circle about the centre B at the distance BC ; in the subduplicate ratio
of AG, the distance of the body from the remoter vertex A of the circle
or rectangular hyperbola, to £AB, the principal semi-diameter of the
figure.

Let AB, the common dia- |t
meter of both figures RPB,

DEB, be bisected in O; and
draw the right line PT that
may touch the figure RPB
in P, and likewise cut that
common diameter AB (pro¬
duced, if need be) in T; and
let SY be perpendicular to
this line, and BQ to this di¬
ameter, and suppose the latus
rectum of the figure RPB to
be L. From Cor. 9, Prop.

XVI, it is manifest that the
velocity of a body, moving
in the line RPB about the
centre S, in any place P, is
to the velocity of a body describing a circle about the same centre, at the
distance SP, in the subduplicate ratio of the rectangle £L X SP to SY 2
For by the properties of the conic sections ACB is to CP 2 as 2AO to L.

2CP 5 X AO

and therefore-rwrr-— is equal to L. Therefore those, velocities ar<

o+

Ak

ACB

to each other in the subduplicate ratio of-

CP 3 X AO X SP
ACB

toSY 2 . Mon

over, by the properties of the conic sections, CO is to BO as BO to Tn
and (by composition or division) as CB to BT. Whence (by division c\
composition) BO —or 4* CO will be to BO as CT to BT, that is, AC

CP 2 X AO X SP

will be to AO as CP to BQ; and therefore- 77^5 -—is equal to

BQ 2 X AC X SP

ACB

AO X BC ‘ Now sn PP ose CP, the breadth of the figure RPB, to

be diminished in infinitum, so as the point P may come to coincide with
the point C, and the point S with the point B, and the line SP with the
line BC, and the line SY with the line BQ; and the velocity of the body
now descending perpendicularly in the line CB will be to the velocity of

11

162

THE MATHEMATICAL PRINCIPLES [BOOK I

a body describing a circle about the centre B, at the distance BC, in thr

subduplicate ratio of

BQ 2 X AC X SP
AO X BC

to SY 2 , that is (neglecting the n\-

tios of equality of SP to BC, and BQ, 2 to SY 2 ), in the subduplicate ratio
of AC to AO, or £AB. Q.E.D.

Cor. 1. When the points B and S come to coincide, TC will become to
TS as AC to AO.

Cor. 2. A body revolving in any circle at a given distance from the
centre, by its motion converted upwards, will ascend to double its distance
from the centre.

PROPOSITION XXXIV. THEOREM X.

If the figure BED is a parabola, I say, that the velocity of a falling
body in any place C is equal to the velocity by which a body may
uniformly describe a circle about the centre B at half the interval BC
For (by Cor. 7, Prop. XVI) the velocity of a
body describing a parabola RPB about the cen¬
tre S, in any place P, is equal to the velocity of
a body uniformly describing a circle about the c
same centre S at half the interval SP. Let the
breadth CP of the parabola be diminished in
infinitum , so as the parabolic arc P/B may come
to coincide with the right line CB, the centre S s
with the vertex B, and the interval SP with the B
interval BC, and the proposition will be manifest. Q.E.D.

PROPOSITION XXXV. THEOREM XI.

The same things supposed, I say, that the area of the figure DES, de¬
scribed by the indefinite radius SD, is equal to the area which a body
xoith a radius equal to half the latus rectum of the figure DES, by
uniformly revolving about the centre S, may describe in the same time\

OF NATURAL PHILOSOPHY.

163

Sec. VII.]

For suppose a body C in the smallest moment of time describes in fal¬
ling the infinitely little line Cc, while another body K, uniformly revolv¬
ing about the centre S in the circle OK/', describes the arc K k. Erect the
perpendiculars CD, cd, meeting the figure DES in D, d. Join SD, Sd.
SK, S k, and draw D d meeting the axis AS in T, and thereon let fall the
perpendicular SY.

Case 1. If the figure DES is a circle, or a rectangular hyperbola, bisect
its transverse diameter AS in O, and SO will be half the latus rectum.
And because TC is to TD as Cc to D d, and TD to TS as CD to SY;
ex cequo TC will be to TS as CD X Cc to SY X T)d. But (by Cor. 1,
Prop. XXXIII) TC is to TS as AC to AO; to wit, if in the coalescence
of the points D, d , the ultimate ratios of the lines are taken. Wherefore
AC is to AO or SK as CD X Cc to S Y X T)d. Farther, the velocity of
the descending body in C is to the velocity of a body describing a circle
about the centre S, at the interval SC, in the subduplicate ratio of AC to
AO or SK (by Prop. XXXIII); and this velocity is to the velocity of a
body describing the circle OKA: in the subduplicate ratio of SK to SC
(by Cor. 6, Prop'IV); and, ex cequo, the first velocity to the last, that is,
the little line Cc to the arc KA', in the subduplicate ratio of AC to SC,
that is, in the ratio of AC to CD. Wherefore CD X Cc is equal to AC
X K/r, and consequently AC to SK as AC X KA; to SY X T)d. and
thence SK X KA; equal to SY X D d, and <!SK X KA; equal to iSY X D d,
that is, the area KSA; equal to the area SDc?. Therefore in every moment
of time two equal particles, KSA; and SDe?, of areas are generated, which,
if their magnitude is diminished, and their number increased in infimhuu,
obtain the ratio of equality, and consequently (by Cor. Lem. IV), the whole
areas together generated are always equal. Q..E.D.

Case 2. But if the figure DES is a
parabola, we shall find, as above, CD X
Cc to SY X D d as TC to TS, that is,
as 2 to 1; and that therefore JCD X Cc
is equal to h SY X T)d. But the veloc¬
ity of the falling body in C is equal to
the velocity with which a circle may be
uniformly described at the interval dSC
(by Prop. XXXIV). And this velocity
to the velocity with which a circle may
be described with the radius SK, that is,
the little line Cc to the arc KA:, is (by
Cor. 6, Prop. IV) in the subduplicate ratio of SK to £SC; that is, in the
ratio of SK to <1CD. Wherefore £SK X KA; is equal to ICD X Cc, and
therefore equal to £SY X T>d ; that is, the area KSA: is equal to the area
SD/Y, as above. Q.E.D.

164

THE MATHEMATICAL PRINCIPLES

[Book 1.

PROPOSITION NXXYI. PROBLEM XXY.

To determine the times of the descent of a body falling from

place A.

Upon the diameter AS, the distance of the body from the
centre at the beginning, describe the semi-circle ADS, as
likewise the semi-circle OKH equal thereto, about the centre
S. From any place C of the body erect the ordinate CD.
Join SD, and make the sector OSK equal to the area ASD.

It is evident (by Prop. XXXY) that the body in falling will
describe the space AC in the same time in which another body,
uniformly revolving about the centre S, may describe the arc
OK. Q.E.F.

a given

PROPOSITION XXXYII. PROBLEM XXYI.

To define the times of the ascent or descent of a body projected upwards
or downwards from a given place.

Suppose the body to go off from the given place G, in the direction of
the line GS, with any velocity. In the duplicate ratio of this velocity to
the uniform velocity in a circle, with which the body may revolve about

the centre S at the given interval SG, take GA to «|AS. If that ratio is
the same as of the number 2 to 1, the point A is infinitely remote; in
which case a parabola is to be described with any latus rectum to the ver¬
tex S, and axis SG ; as appears by Prop. XXXIY. But if that ratio is
less or greater than the ratio of 2 to 1, in the former case a circle, in the
latter a rectangular hyperbola, is to be described on the diameter SA ; as
appears by Prop. XXXIII. Then about the centre S, with an interval
equal to half the latus rectum, describe the circle HA;K; and at the place
G of the ascending or descending body, and at any other place C, erect the
perpendiculars GI, CD, meeting the conic section or circle in I and D.
Then joining SI, SD, let the sectors HSK, HS/c be made equal to the
segments SEIS, SEDS, and (by Prop. XXXY) the body G will describe

Sec. YII.J

OF NATURAL PHILOSOPHY.

165

the space GO in the same time in which the body K may describe the arc
Kk. Q.E.F.

PROPOSITION XXXVIII. THEOREM XII.

Supposing that the centripetal force is proportional to the altitude or
distance of places from the centre, I sap, that the times and velocities
of falling bodies, and the spaces which they describe, are respectively
proportional to the arcs, and the right and versed sines of the arcs.
Suppose the body to fall from any place A in the
right line AS; and about the centre of force S, with
the interval AS, describe the quadrant of a circle AE;
and let CD be the right sine of any arc AD; and the
body A will in the time AD in falling describe the
space AC, and in the place C will acquire the ve¬
locity CD.

This is demonstrated the same way from Prop. X, as Prop. XXX11 was
demonstrated from Prop. XI.

Cor. 1. Hence the times are equal in which one body falling from the
place A arrives at the centre S, and another body revolving describes the
quadrantal arc ADE.

Cor. 2. Wherefore all the times are equal in which bodies falling from
whatsoever places arrive at the centre. For all the periodic times of re¬
volving bodies are equal (by Cor. 3, Prop. IV).

PROPOSITION XXXIX. PROBLEM XXVIT.

Supposing a centripetal force of any kind, and granting the quadra¬
tures of curvilinear figures ; it is required to find the velocity of a body,
ascending or descending in a right line, in the several places through
which it passes ; as also the time in which it will arrive at any place :
and vice versa.

Suppose the body E to fall from any place
A in the right line AD EC; and from its place
E imagine a perpendicular EG always erected p
proportional to the centripetal force in that
place tending to the centre C; and let BFG
be a curve line, the locus of the point G. And D
in the beginning of the motion suppose EG to
coincide with the perpendicular AB; and the
velocity of the body in any place E will be as c
a right line whose square is equal to the cur¬
vilinear area ABGE. QJE.I.

Tn EG take EM reciprocally proportional to

A B

I-T-

T

] 66

THE MATHEMATICAL PRINCIPLES

[Book I

a right line whose square is equal* to the area ABGE, and let YLM be a
curve line wherein the point M is always placed, and to which the right
line AB produced is an asymptote; and the time in which the body in
falling describes the line AE, will be as the curvilinear area ABTYME.
Q.E.I.

For in the right line AE let there be taken the very small line DE of
a given length, and let DLF be the place of the line EMG, when the
body was in D ; and if the centripetal force be such, that a right line,
whose square is equal to the area ABGE, is as the velocity of the descend¬
ing body, the area itself will be as the square of that velocity; that is, if
for the velocities in D and E we write V and Y + I, the area ABFI) will
be as YY, and the area ABGE as YY + 2YI -f II | and by division, the

area DFGE as 2Y1 + II, and therefore

DFGE

DE

will be as-

2YI + II

~ DE

i

that is. if we take the first ratios of those quantities when just nascent, the

2YI

length DF is as the quantity and therefore also as half that quantity

1 X Y
DE

But the time in which the body in falling describes the very

small line DE, is as that line directly and the velocity Y inversely; and
the force will be as the increment I of the velocity directly and the time
inversely; and therefore if we take the first ratios when those quantities
I X Y

are just nascent, as—that is, as the length DF. Therefore a force

proportional to DF or EG will cause the body to descend with a velocity
that is as the right line whose square is equal to the area ABGE. Q.E.D.

Moreover, since the time in which a very small line DE of a given
length may be described is as the velocity inversely, and therefore also
inversely as a right line whose square is equal to the area ABFD ; and
since the line DL, and by consequence the nascent area DLME, will be as
the same right line inversely, the time will be as the area DLME, and
the sum of all the times will be as the sum of all the areas; that is (by
Cor. Lem. IY), the whole time in which the line AE is described will be
as the whole area ATYME. Q.E.D.

Cor. 1. Let P be the place from whence a body ought to fall, so as
that, when urged by any known uniform centripetal force (such as
gravity is vulgarly supposed to be), it may acquire in the place D a
velocity equal to the velocity which another body, falling by any force
whatever, hath acquired in that place D. In the perpendicular DF let
there be taken DR, -which may be o DF as that uniform force to
the other force in the place D. Complete the rectangle PDRQ, and cut
off the area.ABFD equal to that rectangle. Then A will be the place

OF NATURAL PHILOSOPHY.

1(57

Sec. Yll.J

from whence the other body fell. For com¬
pleting the rectangle DRSE, since the area
ABFD is to the area I)FGE as YY to 2YI,
and therefore as £Y to I, that is, as half the
whole velocity to the increment of the velocity
of the body falling by the unequable force; and
in like manner the area PQRD to the area
DRSE as half the whole velocity to the incre¬
ment of the velocity of the body falling by the
uniform force; and since those increments (by
reason of the equality of the nascent times)
are as the generating forces, that is, as the or¬
dinates DF, DR, and consequently as the nascent areas DFGE, DRSE:
therefore, ex aequo , the whole areas ABFD, PQRD will be to one another
as the halves of the whole velocities; and therefore, because the velocities
are equal, they become equal also.

Cor. 2. VYhence if any body be projected either upwards or downwards
with a given velocity from any place D, and there be given the law of
centripetal force acting on it, its velocity will be found in any other place,
as e, by erecting the ordinate eg, and taking that velocity to the velocity
in the place D as a right line whose square is equal to the rectangle
PQRD, either increased by the curvilinear area DF ge, if the place e is
below the place D, or diminished by the same area D Fge, if it be higher,
is to the right line whose square is equal to the rectangle PQRD alone.

Cor. 3. The time is also known by erecting the ordinate em recipro¬
cally proportional to the square root of PQRD -f- or — DFge, and taking
the time in which the body has described the line De to the time in which
another body has fallen with an uniform force from P, and in falling ar¬
rived at D in the proportion of the curvilinear area DL me to the rectan¬
gle 2PD X DL. For the time in which a body falling with an uniform
force hath described the line PD, is to the time in which the same body
has described the line PE in the subduplicate ratio of PD to PE; that is
(the very small line DE being just nascent), in the ratio of PD to PD -f-
4 DE, or 2PD to 2PD -f- DE, and, by division, to the time in which the
body hath described the small line DE, as 2PD to DE, and therefore as
the rectangle 2PD X DL to the area DLME; and the time in which
both the bodies described the very small line DE is to the time in which
the body moving unequably hath described the line De as the area DLME
to the area DLme ; and, ex cequo , the first mentioned of these times is to
the last as the rectangle 2PD X DL to the area DLme.

THE MATHEMATICAL PRINCIPLES

[Book I

168

SECTION VIII.

Of the invention of orbits wherein bodies will revolve, being acted upon
by any sort of centripetal force.

PROPOSITION XL. THEOREM XIII.

[fa, body, acted upon by any centripetal force, is any how moved, and
another body ascends or descends in a right line, and their velocities
be equal in any one case of equal altitudes, their velocities will be also
equed at all equal altitudes.

Let a body descend from A through D and E ; to the centre
O; and let another body move from V in the curve line VIKA:.

From the centre C, with any distances, describe the concentric
circles DI, EK, meeting the right line AC in D and E, and
the curve YIK in I and K. Draw IC meeting KE in N, and
on IK let fall the perpendicular NT; and let the interval DE
or IN between the circumferences of the circles be very small;
and imagine the bodies in D and I to have equal velocities.

Then because the distances CD and Cl are equal, the centri¬
petal forces in D and I will be also equal. Let those forces be k) \\

expressed by the equal lineohe DE and IN; and let the force ’

IN (by Cor. 2 of the Laws of Motion) be resolved into two
others, NT and IT. f l hen the force NT acting in the direction of the
line NT perpendicular to the path ITK of the body will not at all affect
or change the velocity of the body in that path, but only draw it aside
from a rectilinear course, and make it deflect perpetually from the tangent
of the orbit, and proceed in the curvilinear path ITK/j. That whole
force, therefore, will be spent in producing this effect; but the other force
IT, acting in the direction of the course of the body, will be all employed
in accelerating it, and in the least given time will produce an acceleration
proportional to itself. Therefore the accelerations of the bodies in D and
I, produced in equal times, are as the lines DE, IT (if we take the first
ratios of the nascent lines DE, IN, IK, IT, NT); and in unequal times as
those lines and the times conjunctly. But the times in which DE and IK
are described, are, by reason of the equal velocities (in D and I) as the
spaces described DE and IK, and therefore the accelerations in the course
of the bodies through the lines DE and IK are as DE and IT, and DE
and IK conjunctly; that is, as the square of DE to the rectangle IT into
IK. But the rectangle IT X IK is equal to the square of IN, that is,
equal to the square of DE; and therefore the accelerations generated in
the passage of the bodies from D and I to E and K are equal. Therefore
the velocities of the bo lies in E and K are also equal. and by the same
reasoning they will always be found equal in any subsequent equal dis¬
tances. Q.E.D.

Sec. VIll.J

OF NATURAL PHILOSOPHY.

169

By the same reasoning, bodies of equal velocities and equal distances
from the centre will be equally retarded in their ascent to equal distances.
Q.E.D.

Cor. 1. Therefore if a body either oscillates by hanging to a string, or
by any polished and perfectly smooth impediment is forced to move in a
curve line ; and another body ascends or descends in a right line, and their
velocities be equal at any one equal altitude, their velocities will be also
equal at all other equal altitudes. For by the string of the pendulous
body, or by the impediment of a vessel perfectly smooth, the same thing
will be effected as by the transverse force NT. The body is neither
accelerated nor retarded by it, but only is obliged to leave its rectilinear
course.

Cor. 2. Suppose the quantity P to be the greatest distance from the
centre to which a body can ascend, whether it be oscillating, or revolving
in a trajectory, and so the same projected upwards from any point of a
trajectory with the velocity it has in that point. Let the quantity A be
the distance of the body from the centre in any other point of the orbit; and
let the centripetal force be always as the power A n —', of the quantity A, the
index of which power n — 1 is any number n diminished by unity. Then
the velocity in every altitude A will be as y/ P 11 — A 11 , and therefore will
be given. For by Prop. XXXIX, the velocity of a body ascending and
descending in a right line is in that very ratio.

PROPOSITION XLI. PROBLEM XXVIII.

Supposing a centripetal force of any kind, and granting the quadra¬
tures of curvilinear figures, it is required to find as well the trajecto¬
ries in which bodies will move, as the times of their motions in the
trajectories found.

Let any centripetal force tend to A-R

the centre C, and let it be required j-J? - yj -U-—

to find the trajectory VIKA:. Let y\ / \ \/(

there be given the circle VR, described \\i/ D| a/\™\

from the centre C with any interval \V __ L j \ \g

CV; and from the same centre de- /Vy B *1 g\

scribe any other circles ID, KE cut- J Vy / \

ting the trajectory in I and K, and & I ^
the right line CV in D and E. Then V

draw the right line CNIX cutting the c

circles KE, VR in N and X, and the right line CKY meeting tne circle
VR in Y. Let the points I and K be indefinitely near; and let the body
go on from V through I and K to k ; and let the point A be the place
from whence anothe body is to fall, so as in the place D to acquire a ve¬
locity equal to the velocity of the first body in I. And things remaining
as in Prop. XXXIX, the lineola IK, described in the least given time

1.70

THE MATHEMATICAL PRINCIPLES

[Book i

will be as the velocity, and therefore as the right line whose square is
equal to the area ABFD, and the triangle ICK proportional to the time
will be given, and therefore KN will be reciprocally as the altitude IC :
that is (if there be given any quantity Q, and the altitude 1C be called
Q Q,

A), as This quantity — call Z, and suppose the magnitude of Q, to

be such that in some case v/ABFI) may be to Z as IK to KN, and then
in all cases ABFD will be to Z as IK to KN, and ABFI) to ZZ {is
IK 2 to KN 2 , and by division ABFD — ZZ to ZZ as IN 2 to KN 2 , and therc-

_ Q,

fore >/ ABFD — ZZ to Z, or — as IN to KN ; and therefore A X KN

will be equal to

Q X I N

Therefore since YX X XC is to A X KN

as CX 2 , to A A, the rectangle XY X XC will be equal to

q x in x cx . 2

A A v/ABFI) — ZZ!

Therefore in the perpendicular DF let there be taken continually IV, IV

, q q x cx 2 . 1

equal to- - , . . . respectively, anu

2 y/ ABFD — ZZ 2AA y/ ABFD — ZZ

let the curve lines ab , cic, the foci of the points b and c, be described : and
from the point Y let the perpendicular Y a be erected to the line AC, cut¬
ting off the curvilinear areas YDba, YD ca, and let the ordinates Err,
Err, be erected also. Then because the rectangle D6 X IN or D bzD is
equal to half the rectangle A X KN, or to the triangle ICK ; and the
rectangle De X IN or DcrrE is equal to half the rectangle YX X XC, or
to the triangle XCY; that is, because the nascent particles IVsE, ICK
of the areas YDba, VIC are always equal; and the nascent particles
DcrrE, XCY of the areas VDca, YCX are always equal: therefore the
generated area YDba will be equal to the generated area VIC, and there¬
fore proportional to the time; and the generated area Y Dca is equal to
the generated sector YCX. If, therefore, any time be given during which
the body has been moving from Y, there will be also given the area pro¬
portional to it YDba ; and thence will be given the altitude of the body
CD or Cl; and the area YDc«, and the sector YCX equal there’o, together
with its angle YCI. But the angle YCI, and the altitude Cl being given,
there is also given the place I, in which the body will be found at the end
of that time. q.E.I.

Cor. 1. Hence the greatest and least altitudes of the bodies, that is, the
apsides of the trajectories, may be found very readily. For the apsides
are those points in which a right line IC drawn through the centre falls
perpendicularly upon the trajectory YIK; which comes to pass when the
right lines IK and NK become equal; that is, when the area ABFD is
equal to ZZ.

OF NATURAL PHILOSOPHY.

171

Sec. VII 1.1

Cor. 2. So also the angle KIN, in which the trajectory at any place
cuts the line IC. may be readily found by the given altitude 1C of the
body: to wit, by making the sine of that angle to radius as IiN to IK
that is, as Z to the square root of the area ABFD.

Cor. 3. If to the centre C, and the
principal vertex V, there be described a
conic section VRS; and from any point \f
thereof, as R, there be drawn the tangent t
RT meeting the axis CV indefinitely pro¬
duced in the point T; and then joining C
CR there be drawn the right line CP,
equal to the abscissa CT, making an angle VCP proportional to the sector
VCR; and if a centripetal force, reciprocally proportional to the cubes
of the distances of the places from the centre, tends to the centre C; and
from the place V there sets out a body with a just velocity in the direc¬
tion of a line perpendicular to the right,line CV; that body will proceed
in a trajectory VPQ, which the point P will always touch; and therefore
if the conic section VRS be an hyberbola, the body will descend to the cen¬
tre ; but if it be an ellipsis, it will ascend perpetually, and go farther and
farther off in infinitum. And, on the contrary, if a body endued with any
velocity goes off from the place V, and according as it begins either to de*
scend obliquely to the centre, or ascends obliquely from it, the figure VRS
be either an hyperbola or an ellipsis, the trajectory may be found by increas¬
ing or diminishing the angle VCP in a given ratio. And the centripetal
force becoming centrifugal, the body will ascend obliquely in the trajectory
VPQ, which is found by taking the angle VCP proportional to the elliptic
sector VRC, and the length CP equal to the length CT, as before. All these
things follow from the foregoing Proposition, by the quadrature of a certain
curve, the invention of which, as being easy enough, for brevity’s sake I omit.

PROPOSITION XLII. PROBLEM XXIX.

The law of centripetal force being given , it is required to find the motion
of a body setting out from a given place, with a given velocity , in the

direction of a given right line.

Suppose the same things as in
the three preceding propositions;
and let the body go off from
the place I in the direction of the '
little line, IK, with the same ve¬
locity as another body, by falling
with an uniform centripetal force
from the place P, may acquire in
1); and let this uniform force be
to the force with which the body

172

THE MATHEMATICAL PRINCIPLES

[Book 1.

is at first urged in I, as DR to DF. Let the body go on towards k; and
about the centre C, with the interval C k, describe the circle he, meeting
the right line PD in e, and let there be erected the lines eg, ev, ew, ordi-
nately applied to the curves BFg - , abv, acio. From the given rectangle
PDRQ, and the given law of centripetal force, by which the first body is
acted on, the curve line BFg is also given, by the construction of Prop.
XXVII, and its Cor. 1. Then from the given angle CIK is given the
proportion of the nascent lines IK, KN; and thence, by the construction
of Prob. XXVIII, there is given the quantity Q, with the curve lines abv,
acw; and therefore, at the end of any time D bve, there is given both
the altitude of the body Ce or C k, and the area D ewe, with the sector
equal to it XC y, the angle ICk, and the place k, in which the body will
then be found. Q.E.I.

We suppose in these Propositions the centripetal force to vary in its
recess from the centre according to some law, which any one may imagine
at pleasure; but at equal distances from the centre to be everywhere the
same.

I have hitherto considered the motions of bodies in immovable orbits.
It remains now to add something concerning their motions in orbits which
revolve round the centres of force.

SECTION IX.

Of the motion of bodies in moveable orbits ; and of the motion of the

apsides.

PROPOSITION XLIII. PROBLEM XXX.
h is required to make a body move in a trajectory that revolves about
the centre of force in the same manner as another body in the same
trajectory at rest.

In. the orbit VPK, given by position, let the body
P revolve, proceeding from V towards K. From
the centre C let there be continually drawn Cp, equal
to CP, making the angle VC p proportional to the
angle VCP; and the area which the line Cp describes
will be to the area VCP, which the line CP describes
at the same time, as the velocity of the describing
line Cp to the velocity of the describing line CP;
that is, as the angle VC 'p to the angle VCP, therefore in
and therefore proportional to the time. Since, then, the area described by
the line Cp in an immovable plane is proportional to the time, it is manifest
that a body, being acted upon by a just quantity of centripetal force may

given ratio,

Sec. LX.]

OF NATURAL PH1LUSOPHY.

175

revolve with the point p in the curve line which the same point p, by the
method just now explained, may be made to describe an immovable plane.
Make the angle YCw equal to the angle PCp, and the line C a equal to
CY, and the figure uGp equal to the figure YCP, and the body being al¬
ways in the point p , will move in the perimeter of the revolving figure
uGp, and will describe its (revolving) arc up in the same time tha* the
other body P describes the similar and equal arc YP in the quiescov.t fig¬
ure YPK. Find, then, by Cor. 5, Prop. YI., the centripetal force by vrhich
the body may be made to revolve in the curve line which the point p de¬
scribes in an immovable plane, and the Problem will be solved. <AE.F.

PROPOSITION NLIV. THEOREM XIY.

The difference of the forces , by which two bodies may be math. to move
equally , one in a quiescent , the other in the same orbit revolving , i t in
a triplicate ratio of their common altitudes inversely.

Let the parts of the quiescent or¬
bit YP, PK be similar and equal to
the parts of the revolving orbit up,
pk ; and let the distance of the points
P and K be supposed of the utmost
smallness Let fall a perpendicular
kr from the point k to the right line
pC, and produce it to m, so that mr
may be to kr as the angle YC 'p to the
angle YCP. Because the altitudes
of the bodies PC and pG, KC and
kC, are always equal, it is manifest
that the increments or decrements of
the lines PC and pC are always
equal; and therefore if each of the
several motions of the bodies in the places P and p be resolved into two
(by Cor. 2 of the Laws of Motion), one of which is directed towards the
centre, or according to the lines PC, pC, and the other, transverse to the
former, hath a direction perpendicular to the lines PC and pC ; the mo¬
tions towards the centre will be equal, and the transverse motion of the
body p will be to the transverse motion of the body P as the angular mo¬
tion of the line pG to the angular motion of the line PC; that is, as the
angle YGp to the angle YCP. Therefore, at the same time that the bodv
P, by both its motions, comes to the point K, the body p, having an equal
motion towards the centre, will be equally moved from p towards C ; and
therefore that time being expired, it will be found somewhere in the
line rnkr, which, passing through the point k, is perpendicular to the line
pG ; and by its transverse motion will acquire a distance from the line

174

THE MATHEMATICAL PRINCIPLES

[Book 1.

vC, that will be to the distance which the other body P acquires from the
line PC as the transverse motion of the body p to the transverse motion of
the other body P. Therefore since kr is equal to the distance which the
body P acquires from the line PC, and m,r is to kr as the angle VC p to
the angle VCP, that is, as the transverse motion of the body p to the
transverse motion of the body P, it is manifest that the body p, at the ex¬
piration of that time, will be found in the place m. These things will be
so, if the bodies p and P are equally moved in the directions of the lines
pC and PC, and are therefore urged with equal forces in those directions,
h ut if we take an angle pCn that is to the angle pOk as the angle VC p
to the angle VCP, and nC be equal to kC, in that case the body p at the
expiration of the time will really be in n ; and is therefore urged with a
greater force than the body P, if the angle nOp is greater than the angle
kOp, that is, if the orbit vpk, move either in consequentia, or in antece-
dentici , with a celerity greater than the double of that with which the line
CP moves in conseqnentia ; and with a less force if the orbit moves slower
in antecedentia. And ihe difference of the forces will be as the interval
mn of the places through which the body would be carried by the action of
that difference in that given space of time. About the centre C with the
interval C n or C k suppose a circle described cutting the lines mr, inn pro¬
duced in s and t, and the rectangle mn X mt will be equal to the rectan-

mk X ms

gle mk X ms , and therefore mn will be equal to-——. But since

the triangles pOk, pCn, in a given time, are of a given magnitude, kr and
mr, a id their difference mk , and their sum ms, are reciprocally as the al¬
titude pC, and therefore the rectangle mk X ms is reciprocally as the
square of the altitude pC. But, moreover, mt is directly as \mt, that is, as
the altitude pC. These are the first ratios of the nascent lines; and hence
mk X ms

- r —- that is, the nascent lineola mn, and the difference of the forces

mt

proportional thereto, are reciprocally as the cube of the altitude pC.

Q.E.D.

Cor. 1. Hence the difference of the forces in the places P and p, or K and
k, is to the force with which a body may revolve with a circular motion
from R to K, in the same time that the body P in an immovable orb de¬
scribes the arc PK, as the nascent line mn to the versed sine of the nascent
mk X ms rk ^

arc RK, that is, as-—— to or as mk X ms to the square of

rk ; that is, if we take given quantities F and G in the same ratio to one
another as the angle VCP bears to the angle VC p, as GG — FF to FF.
And, therefore, if from the centre C, with any distance CP or Op, there be
described a circular sector equal to the whole area VPC, which the body

Sec. IX.]

OF NATURAL PHILOSOPHY.

175

revolving in an immovable orbit has by a radius drawn to the centre de¬
scribed in any certain time, the difference of the forces, with which the
body P revolves in an immovable orbit, and the body p in a movable or¬
bit, will be to the centripetal force, with which another body by a radius
drawn to the centre can uniformly describe that sector in the same time
as the area VPC is described, as GG— FF to FF. For that sector and
the area pOk are to one another as the times in which they are described.

Cor. 2. If the orbit YPK be an
ellipsis, having its focus C, and its
highest apsis Y, and we suppose the
the ellipsis upk similar and equal to
it, so that pC may be always equal
to PC, and the angle YCjo be to the
angle YCP in the given ratio of G
to F ; and for the altitude PC or pC
we put A, and 2R for the latus rec¬
tum of the ellipsis, the force with
which a body may be made to re¬
volve in a movable ellipsis will be as

FF RGG — RFF
AA + A 5 5

and vice versa.

1 iCt the force with which a body may

revolve in an immovable ellipsis be expressed by the quantity

FF

AA’

and the

FF

force in Y will be ^ Trr . But the force with which a body mav revolve in
CY 2 J

a circle at the distance CY, with the same velocity as a body revolving in
an ellipsis has in Y, is to the force with which a body revolving in an ellip¬
sis is acted upon in the apsis Y, as half the latus rectum of the ellipsis to the

RFF

semi-diameter CY of the circle, and therefore is as : and the force

CY J

which is to this, as GG

RGG —RFF , „

FF to FF, is as--: and this force

’ CY 3

(by Cor. 1 of this Prop.) is the difference of the forces in Y, with which the
body P revolves in the immovable ellipsis YPK, and the body p in the
movable ellipsis upk. Therefore since by this Prop, that difference ;it

any other altitude A is to itself at the altitude CY as — to the same

.. . • RGG — RFF

uiflerence in every altitude A will be as -

A 3

Therefore to the

FF

force ^ by which the body may revolve in an immovable ellipsis VTK

176

THE MATHEMATICAL PRINCIPLES

[Book I.

add the excess

and the sum will be the whole force

Viiv j WiiU mv pum TV 111 k/V UUL UUV1V JIUltL YA

RGG—RFF t t J , . , . . ,

-- by which a body may revolve m the same time m the mot-

able ellipsis upk.

Cor. 3. In the same manner it will be found, that, if the immovable or¬
bit VPK be an ellipsis having its centre in the centre of the forces C, and
there be supposed a movable ellipsis upk, similar, equal, and concentrical
to it; and 2R be the principal latus rectum of that ellipsis, and *2T the
latus transversum, or greater axis; and the angle VC p be continually to the
angle VCP as G to F; the forces with which bodies may revolve in the im-

FFA FFA

movable and movable ellipsis, in equal times, will be as —^ - and -y^

- ill X , -

-f- —- respectively.

Cor. 4. And universally, if the greatest altitude CV of the body be called
T, and the radius of the curvature which the orbit VPK has in V, that is,
the radius of a circle equally curve, be called R, and the centripetal force
with which a body may revolve in any immovable trajectory VPK at the place
VFF

V be called , and in other places P be indefinitely styled X ; and the

altitude CP be called A, and G be taken to F in the given ratio of the
angle VC p to the angle VCP; the centripetal force with which the same
body will perform the same motions in the same time, in the same trajectory
upk revolving with a circular motion, will be as the sum of the forces X 4*
VRGG — VRFF
A 3

Cor. 5. Therefore the motion of a body in an immovable orbit being
given, its angular motion round the centre of the forces may be increased
or diminished in a given ratio; and thence new immovable orbits may be
found in which bodies may revolve with new centripetal forces.

Cor. 6. Therefore if there be erected the line VP of an indeterminate
p length, perpendicular to the line CV given by po-

— v sition, and CP be drawn, and Cp equal to it, mak-
ing the angle VC p having a given ratio to the an-
/ \ gle VCP, the force with which a body may revolve

\ in the curve line Ypk, which the point p is con-

/ tinually describing, will be reciprocally as the cube

** C of the altitude Cp. For the body P, by its vis in¬

ertia alone, no other force impelling it, will proceed uniformly in the right
line VP. Add, then, a force tending to the centre C reciprocally as the
cube of the altitude CP or Cp, and (by what was just demonstrated) the

OF NATURAL PHILOSOPHY.

1 77*

Sec. IX.J

body will deflect from the rectilinear motion into the curve line Yplc. But
this curve V pk is the same with the curve ^PQ found in Cor. 3, Prop
XLI, in which, I said, bodies attracted with such forces would ascend
obliquely.

PROPOSITION XLY. PROBLEM XXXI.

To find the motion of the apsides in orbits approaching very near to

circles.

This problem is solved arithmetically by reducing the orbit, which a
body revolving in a movable ellipsis (as in Cor. 2 and 3 of the above
Prop.) describes in an immovable plane, to the figure of the orbit whose
apsides are required ; and then seeking the apsides of the orbit which that
body describes in an immovable plane. But orbits acquire the same figure,
if the centripetal forces with which they are described, compared between
themselves, are made proportional at equal altitudes. Let the point Y be
the highest apsis, and write T for the greatest altitude CY, A for any other
altitude CP or Cp, and X for the difference of the altitudes CY — CP;
and the force with which a body moves in an ellipsis revolving about its

pipi RGGr_ rfF

focus C (as in Cor. 2), and which in Cor. 2 was as H--,

that is as,

FFA + RGG — RFF
A 3

, by substituting T — X for A, will be-

RGG — RFF + TFF — FFX

come as --

A 3

In like manner any other cen¬

tripetal force is to be reduced to a fraction whose denominator is A 3 , and
the numerators are to be made analogous by collating together the homo¬
logous terms. This will be made plainer by Examples.

Example 1. Let us suppose the centripetal force to be uniform,
A 3

and therefore as or, writing T — X for A in the numerator, as

T 3 — 3TTX + 3TXX — X 3 ^ „ .

_= :s= _ — . Then collating together the correspon-

A 3

dent terms of the numerators, that is, those that consist of given quantities,
with those of given quantities, and'those of quantities not given with those
of quantities not given, it will become RGG — RFF + TFF to T 3 as —
FFX to 3TTX + 3TXX — X 3 , or as —FF to —3TT + 3TX — XX.
Now since the orbit is supposed extremely near to a circle, let it coincide
with a circle; and because in that case R and T become equal, and X is
infinitely diminished, the last ratios will be, as RGG to T 2 , so — FF to —
3TT, or as GG to TT, so FF to 3TT; and again, as GG to FF, so TT
to 3TT, that is, as 1 to 3 ; and therefore G is to F, that is, the angle YC p
to the angle YCP, as 1 to 3. Thereiore since the body, in an immovable

'2

J7S

THE MATHEMATICAL PRINCIPLES

[Book I

ellipsis, in descending from the upper to the lower apsis, describes an angle,
if I may so speak, of 180 deg., the other body in a movable ellipsis, and there¬
fore in the immovable orbit we are treating of, will in its descent from

180

the upper to the lower apsis, describe an angle YCp of —~ deg. And this

x/o

comes to pass by reason of the likeness of this orbit which a body acted
upon by an uniform centripetal force describes, and of that orbit which a
body performing its circuits in a revolving ellipsis will describe in a quies¬
cent plane. By this collation of the terms, these orbits are made similar;
not universally, indeed, but then only when they approach very near to a
circular figure. A body, therefore revolving with an uniform centripetal

ISO

force in an orbit nearly circular, will always describe an angle of — ~ deg/, or

\/0

103 deg., 55 m., 23sec., at the centre; moving from the upper apsis to the
lower apsis when it has once described that angle, and thence returning to
the upper apsis when it has described that angle again; and so on in in¬
finitum.

Exam. 2. Suppose the centripetal force to be as any power of the alti-

A n

tude A, as, for example, A n — 3 , or — 3 ; where n — 3 and n signify any in¬
dices of powers whatever, whether integers or fractions, rational or surd,
affirmative or negative. That numerator A n or T — X| n being reduced to
an indeterminate series by my method of converging series, will become

T n — ??XT n -

XXT n — 2 , (fee. And conferring these terms

with the terms of the other numerator RGG — RFF + TFF — FFX, it

becomes as RGG —RFF + TFF to T", so — FF to — »,T"—’ + —~

XT n — 2 , cfec. And taking the last ratios where the orbits approach to
circles, it becomes as RGG to T‘\ so — FF to — nT* 1 — T , or as GG to
T n — T , so FF to ?iT n — ; and again. GG to FF, so T n — 1 to n'Y n — l , that
is, as 1 to n ; and therefore G is to F, that is the angle YCp to the angle
YCP, as 1 to sfn. Therefore since the angle YCP, described in the de¬
scent of the body from the upper apsis to the lower apsis in an ellipsis, is
of ISO deg., the angle YCp, described in the descent of the body from the
upper apsis to the lower apsis in an orbit nearly circular which a body de¬
scribes with a centripetal force proportional to the power A"— 3 , will be equal
ISO

to an angle of-deg., and this angle being repeated, the body will re¬

's/ n

turn from the lower to the upper apsis, and so on in infinitum. As if the
centripetal force be as the distance of the body from the centre, that is, as A,
A 4

or -7-j, n will be equal to 4, and y/n equal to 2 ; and therefore the angle
A

Sec. IX.]

OF NATURAL PHILOSOPHY.

between the upper and the lower apsis will be equal to — deg., or 90 deg.

/w

Therefore the body having performed a fourth part of one revolution, will
arrive at the lower apsis, and having performed another fourth part, will
arrive at the upper apsis, and so on by turns in infinitum. This appears
also from Prop. X. For a body acted on by this centripetal force will re¬
volve in an immovable ellipsis, whose centre is the centre of force. If the

1 A 2

centripetal force is reciprocally as the distance, that is, directly as — or ^

n will be equal to 2; and therefore the angle between the upper and lower
180

apsis will be —- deg., or 1 27 deg., 16 min., 45 sec.; and therefore a body re-

v/2

volving with such a force, will by a perpetual repetition of this angle, move
alternately from the upper to the lower and from the lower to the upper
apsis for ever. So, also, if the centripetal force be reciprocally as the
biquadrate root of the eleventh power of the altitude, that is, reciprocally

as A and, therefore, directly as -~ v or as —, n will be equal to [, and

ISO

— deg. will be equal to 360 deg.; and therefore the body parting from

the upper apsis, and from thence perpetually descending, will arrive at the
lower apsis when it has completed one entire revolution; and thence as¬
cending perpetually, when it has completed another entire revolution, it
will arrive again at the upper apsis; and so alternately for ever.

Exam. 3. Taking m and n for any indices of the powers of the alti¬
tude, and b and c for any given numbers, suppose the centripetal force

_ bA m -f cA" . b into T — Xl m -f- c into T — X|"

to be as--, that is, as- t-s-

A 3 A 3

or (by the method of converging series above-menticncd) as

AT‘ n + cT n — mbXT n — 1 //cXT n — 1 mm — m^ _ 2 _j_ vH '— 7/

eXXT"

and comparing the terms of the numerators, there will

arise RGG — RFF -j- TFF to &T m + cT n as — FF to — w-Z>T r

bXT m — 2 +

cX T n — 2 , (fee. And tak-

ing the last ratios that arise when the orbits come to a circular form, there
will come forth GG to 6T m — 1 4* cT n — I aa FF to mbT m — 1 + wcT n — 1 ;
and again, GG to FF as 6T m — 1 + cT n — 1 to mbT° — 1 + ncT n — *.
/This proportion, by expressing the greatest altitude CV or T arithmeti¬
cally by unity, becomes, GG to FF as b -j- c to mb 4 wc, and therefore as 1

(80

THE MATHEMATICAL PRINCIPLES

[Book ]

mb -f- vc

to - Whence G becomes to F, that is, the angle VCp to the an-

b -f c
gle VCP, as 1 to y

mb + nc

~b+Y-

And therefore since the angle VCP between

the upper and the lower apsis, in an immovable ellipsis, is of 180 deg., the
angle VC p between the same apsides in an orbit which a body describes

bA™ -j- c A n

with a centripetal force, that is, as-—, will be equal to an angle of

ISO — r~Z“ ; deg. And by the same reasoning, if the centripetal force

be as

mb + vc
bA m — cA"

A 3

, the angle between the apsides will be found equal to

After the same manner the Problem is solved in

more difficult cases. The quantity to which the centripetal force is pro¬
portional must always be resolved into a converging series whose denomi¬
nator is A 3 . Then the given part of the numerator arising from that
operation is to be supposed in the same ratio to that part of it which is not
given, as the given part of this numerator RGG — RFF + TFF — FFX
is to that part of the same numerator which is not given. And taking
away the superfluous quantities, and writing unity for T, the proportion
of G to F is obtained.

Cor. 1. Hence if the centripetal force be as any power of the altitude,
that power may be found from the motion of the apsides; and so contra¬
riwise. That is, if the whole angular motion, with which the body returns
to the same apsis, be to the angular motion of one revolution, or 360 deg.,
as any number as m to another as n, and the altitude called A; the force

nn

will be as the power A nun ® of the altitude A; the index of which power is

—— — 3. This appears by the second example. Hence it is plain that

the force in its recess from the centre cannot decrease in a greater than a
triplicate ratio of the altitude. A body revolving with such a force / and
parting from the apsis, if it once begins to descend, can never arrive at the
lower apsis or least altitude, but will descend to the centre, describing the
curve line treated of in Cor. 3, Prop. XLI. But if it should, at its part-
i ng from the lower apsis, begin to ascend never so little, it will ascend in
infinitum , and never come to the upper apsis; but will describe the curve
line spoken of in the same Cor., and Cor. 6, Prop. XLIV. So that where
the force in its recess from the centre decreases in a greater than a tripli¬
cate ratio of the altitude, the body at its parting from the apsis, will either
descend to the centre, or ascend in infinitum , according as it descends or
ascends at the beginning of its motion. But if the force in its recess from

'Sec. IX.J

OF NATURAL PHILOSOPHY.

1S1

the centre either decreases in a less than a triplicate ratio of the altitude,
or increases in any ratio of the altitude whatsoever, the body will never
descend to the centre, but will at some time arrive at the lower apsis; and,
on the contrary, if the body alternately ascending and descending from one
apsis to another never comes to the centre, then either the force increases
in the recess from the centre, or it decreases in a less than a triplicate ratio
of the altitude; and the sooner the body returns from one apsis to another,
the farther is the ratio of the forces from the triplicate ratio. As if the
body should return to and from the upper apsis by an alternate descent and
ascent in 8 revolutions, or in 4, or 2, or ; that is, if m should be to n as 8,

or 4, or 2, or to 1, and therefore --3,be g 1 ,— 3,or T \ — 3,or{ — 3,or

3; then the force will be as A 6 4 3 ’ or A 18 3 ’ or A 4 3? or A 9

or A 3 ™ 1 ™ or A 3 ”** or A 3

that is, it will be reciprocally as A
If the body after each revolution returns to the same apsis, and the apsis

nn _ 3

remains unmoved, then m will be to n as 1 to 1, and therefore A^i
will be equal to A

, or -7—7-; and therefore the decrease of the forces will
’ AA ’

be in a duplicate ratio of the altitude; as was demonstrated above. If the
body in three fourth parts, or two thirds, or one third, or one fourth part
of an entire revolution, return to the same apsis; m will be to n as | or \

or ^ or l to 1, and therefore Amm 3 is equal to A 9 ; or A 4 or A

_ 3 18 _ 3 . 1_1

' ’ or A ; and therefore the force is either reciprocally as A 0 or

3 6 13

A 4 ’ or directly as A or A . Lastly if the body in its progress from the
upper apsis to the same upper apsis again, goes over one entire revolution
and three deg. more, and therefore that apsis in each revolution of the body
moves three deg. in consequentia ; then m will be to n as 363 deg. to

360 deg. or as 121 to 120, and therefore Amm will be equal to

_ 2 9 5 2 3

A 146415 and therefore the centripetal force will be reciprocally as

2 9 5 2 3 _ 2 _ 4 _

A J 4 6 4 1’ or reciprocally as A 2 4 3 very nearly. Therefore the centripetal
force decreases in a ratio something greater than the duplicate; but ap¬
proaching 59f times nearer to the duplicate than the triplicate.

Cor. 2. Hence also if a body, urged by a centripetal force which is re¬
ciprocally as the square of the altitude, revolves in an ellipsis whose focus
is in the centre of the forces; and a new and foreign force should be added
to or subducted from this centripetal force, the motion of the apsides arising
from that foreign force may (by the third Example) be known; and so on
the contrary. As if the force with which the body revolves in the ellipsis

182

THE MATHEMATICAL PRINCIPLES

[Book I

DG aS AA ’ an ^ ^ ore ^ n f° rce subducted as cA, and therefore the remain-

ing force as —-; then (by the third Example) b will be equal to 1 .

tn equal to 1 , and n equal to 4; and therefore the angle of revolution be

| _ Q

-tween the apsides is equal to 180 ^ deg. Suppose that foreign force

to be 357.45 parts les 3 than the other force with which the body revolves
in the ellipsis ; that is, c to be 3 T ; A or T being equal to 1; and then
1 — c

180will be 180-y/f jf|-f or 180.7623, that is, 180 deg., 45 min.,

44 sec. Therefore the body, parting from the upper apsis, will arrive at
the lower apsis with an angular motion of 180 deg., 45 min., 44 sec, and
this angular motion being repeated, will return to the upper apsis; and
therefore the upper apsis in each revolution will go forward 1 deg., 31 min.,
2 S sec. The apsis of the moon is about twice as swift

So much for the motion of bodies in orbits whose planes pass through
the centre of force. It now remains to determine those motions in eccen¬
trical planes. For those authors who treat of the motion of heavy bodies
used to consider the ascent and descent of such bodies, not only in a per¬
pendicular direction, but at all degrees of obliquity upon any given planes ;
and for the same reason we are to consider in this place the motions of
bodies tending to centres by means of any forces whatsoever, when those
bodies move in eccentrical planes. These planes are supposed to be
perfectly smooth and polished, so as not to retard the motion of the bodies
in the least. Moreover, in these demonstrations, instead of the planes upon
which those bodies roll or slide, and which are therefore tangent planes to
the bodies, I shall use planes parallel to them, in which the centres of the
bodies move, and by that motion describe orbits. And by the same method
I afterwards determine the motions of bodies performed in curve superficies.

SECTION X.

Of the motion of bodies in given superficies, and of the reciprocal motion
offunependulous bodies.

PROPOSITION XLYI. PROBLEM XXXII.

Any kind of centripetal force being supposed, and the centre offorce, and
any plane whatsoever in which the body revolves, being given , and the
quadratures of curvilinear figures being allowed ; it is required to de¬
termine the motion of a body going off from a given place ., with a
given velocity, in the direction of a given right line in that plane.

Sec. X.J of natural philosophy. 183

Let S be the centre of force, SC the
least distance of that centre from the given
plane, P a body issuing from the place P
in the direction of the right line PZ, Q
the same body revolving in its trajectory,
and PQR the trajectory itself which is
required to be found, described in that
given plane. Join CQ, QS, and if in QS
we take SV proportional to the centripetal
force with which the body is attracted to¬
wards the centre S, and draw VT parallel
to CQ, and meeting SC in T; then will the force SV be resolved into
two (by Cor. 2, of the Laws of Motion), the force ST, and the force TV ; of
which ST attracting the body in the direction of a line perpendicular to
that plane, does not at all change its motion in that plane. But the action
(f the other force TV, coinciding with the position of the plane itself, at¬
tracts the body directly towards the given point C in that plane; and
t lerefcre causes the body to move in this plane in the same manner as if
the force S T were taken away, and the body were to revolve in free space
about the centre C by means of the force TV alone. But there being given
the centripetal force TV with which the body Q revolves in free space
about the given centre C, there is given (by Prop. XLII) the trajectory
PQR which the body describes; the place Q, in which the body will be
found at any given time; and, lastly, the velocity of the body in that place
Q. And so e contra. Q.E.I.

PROPOSITION XLVII. THEOREM XV.

Supposing the centripetal force to he proportional to the distance of the
body from, the centre ; all bodies revolving i?i any planes whatsoever
will describe ellipses , and complete their revolutions in equal times ;
and those which move in right lines , running backwards and forwards
alternately , will complete their several periods of going and returning
in the same times.

For letting all things stand as in the foregoing Proposition, the force
SV, with -which the body Q revolving in any plane PQR is attracted to¬
wards the centre S, is as the distance SQ ; and therefore because SV and
SO, TV and CQ, are proportional, the force TV with which the body is
attracted towards the given point C in the plane of the orbit is as the dis¬
tance CQ. Therefore the forces with which bodies found in the plane
PQR are attracted towaids the point C, are in proportion to the distances
equal to the forces with which the same bodies are attracted every way to¬
wards the centre S ; and therefore the bodies will move in the same times,
and in the same figures, in any plane PQR about the point C. as they

184

THE MATHEMATICAL PRINCIPLES

[Book 1.

would do in free spaces about the centre S; and therefore (by Cor. 2, Prop.
Xj ai d Gor. 2, Prop. XXXVIII.) they will in equal times either describe
ellipsis in that plane about the centre C, or move to and fro in right lines
passing through the centre C in that planej completing the same periods
of time in all cases. Q.E.D.

SCHOLIUM.

Tne ascent and descent of bodies in curve superficies has a near relation
to these motions we have been speaking of. Imagine curve lines to be de¬
scribed on any plane, and to revolve about any given axes passing through
the centre of force, and by that revolution to describe curve superficies ; and
that the bodies move in such sort that their centres may be always found
in those superficies. If those bodies reciprocate to and fro with an oblique
ascent and descent, their motions will be performed in planes passing through
tlie axis, and therefore in the curve lines, by whose revolution those curve
superficies were generated. In those cases, therefore, it will be sufficient to
consider the motion in those curve lines.

PROPOSITION XLVIII. THEOREM XVI.

If « wheel stands upon the outside of a globe at right angles thereto, and
revolving about its own axis goes forward in a great circle, the length
of the curvilinear path which any point, given in the perimeter of the
wheel , hath described, since the time that it touched the globe [which
curvilinear path we may call the cycloid, or epicycloid), will be to double
the versed sine of half the arc which since that time has touched the
globe in passittg over it, as the sum of the diameters of the globe and
the wheel to the semi-diameter of the globe.

PROPOSITION XLIX. THEOREM XVII.

If a wheel stand upon the inside of a concave globe at right angles there¬
to, and revolving about its own axis go forward in one of the great
circles of the globe, the length of the curvilinear path which any point,
given in the perimeter of the wheel\ hath described since it touched the
globe, will be to the double of the versed sine of half the arc which in
all that time has touched the globe in passing over it, as the difference
of the diameters of the globe and the wheel to the semi-diameter of the
globe.

Let ABL be the globe, C its centre, BPV the wheel insisting thereon,
E the centre of the wheel, B the point of contact, and P the given point
in the perimeter of the wheel. Imagine this wheel to proceed in the great
circle ABL from A through B towards L, and in its progress to revolve in
such a manner that the arcs AB, PB may be always equal one to the other,
and the given point P in the perimeter of the wheel may describe in thf

Sec. X.I

OF NATURAL PHILOSOPHY.

1S5

s

mean time the curvilinear path AP. Let AP be the whole curvilinear
path described since the wheel touched the globe in A, and the length of
this path AP will be to twice the versed sine of the arc |d?B as 2CE to
CB. For let the right line CE (produced if need be) meet the wheel in V,
and join CP, BP, EP, VP; produce CP, and let fall thereon the perpen¬
dicular VF. Let PH, VH, meeting in II, touch the circle in P and V,
and let PH cut VF in G, and to VP let fall the perpendiculars GI, HK.
From the centre C with any interval let there be described the circle nom,
cutting the right line CP in n, the perimeter of the wheel BP in o, and
the curvilinear path AP in m ; and from the centre V with the interval
Vo let there be described a circle cutting VP produced in q.

Because the wheel in its progress always revolves about the point of con¬
tact B, it is manifest that the right line BP is perpendicular to that curve line
AP which the point P of the wheel describes, and therefore that the right
line VP will touch this curve in the point P. Let the radius of the circle nom
be gradually increased or diminished so that at last it become equal to the
distance CP; and by reason of the similitude of the evanescent figure
P nnmq, and the figure PFGVI, the ultimate ratio of the evanescent lineola;
Pm, Pn, Po, P q, that is, the ratio of the momentary mutations of the curve
AP, the right line CP, the circular arc BP, and the right line VP, will W

THE MATHEMATICAL PRINCIPLES

[Book 1.

1S6

the same as of the lines PY, PF, PG ; PI, respectively. But since VF is
perpendicular to CF, and YH to CY, and therefore the angles HVG, YCF
equal; and the angle VHG (because the angles of the quadrilateral figure
HYEP are right in Y and P) is equal to the angle CEP, the triangles
VHG, CEP will be similar; and thence it will come to pass that asEP is
to CE so is HG to HY or HP, and so KI to KP, and by composition or
division as CB to CE so is PI to PK, and doubling the consequents asCB
to 2CE so PI to PY, and so is P q to P m. Therefore the decrement of the
line YP, that is, the increment of the line BY—YP to the increment of the
curve line AP is in a given ratio of CB to 2CE, and therefore (by Cor.
Lem. IV) the lengths BY—YP and AP, generated by those increments, are
in the same ratio. But if BY be radius, YP is the cosine of the angle BYP
or JBEP, and therefore BY—YP is the versed sine of the same angle, and
therefore in this wheel, whose radius is ^BV, BY—YP will be double the
versed sine of the arc ^BP. Therefore AP is to double the versed sine of
the arc ^BP as 2CE to CB. Q.E.D.

The line AP in the former of these Propositions we shall name the cy¬
cloid without the globe, the other in the latter Proposition the cycloid within
the globe, for distinction sake.

Cor. 1. Hence if there be described the entire cycloid ASL, and the
same be bisected in S, the length of the part PS will be to the length PY
(which is the double of the sine of the angle YBP, when EB is radius) as
2CE to CB, and therefore in a given ratio.

Cor. 2. And the length of the semi-perimeter of the cycloid AS will be
equal to a right line which is to the dumeter of the wheel BY as 2CF-
to CB.

PROPOSITION L. PROBLEM XXXIII.

To cause a pendulous body to oscillate in a given cycloid.

Let there be given within the globe QVS de¬
scribed with the centre C, the cycloid QRS, bi¬
sected in R, and meeting the superficies of the
globe with its extreme points Q and S on either
hand. Let there be drawn CR bisecting the arc
QS in O, and let it be produced to A in such
sort that CA may be to CO as CO to CR.
About the centre C, with the interval CA, let
there be described an exterior globe UAF ; and
within this globe, by a wheel whose diameter is
AO, let there be described two semi-cycloids AQ,
AS, touching the interior globe in Q, and S, and meeting the exterior globe
in A. From that point A, with a thread APT in length equal to the line
AR, let the body T depend, and oscillate in such manner between the two

Skc. X.J

OF NATURAL PHILOSOPHY.

187

semi-cycloids AQ, AS, that, us often as the pendulum parts from the per¬
pendicular AR, the upper part of the thread AP may be applied to that
semi-cycloid APS towards which the motion tends, and fold itself round
that curve line, as if it were some solid obstacle, the remaining part of the
same thread PT which has not yet touched the semi-cycloid continuing
straight. Then will the weight T oscillate in the given cycloid QRS.
Q.E.F.

For let the thread PT meet the cycloid QRS in T, and the circle QOS
m Y, and let OY be drawn; and to the rectilinear part of the thread PT
from the extreme points P and T let there be erected the perpendiculars
BP, TW, meeting the right line CV in B and W. It is evident, from the
construction and generation of the similar figures AS, SR, that those per¬
pendiculars PB, TYV, cut off from CV the lengths YB, YYV equal the
diameters of the wheels OA, OR. Therefore TP is to VP (which is dou¬
ble the sine of the angle YBP when |BY is radius) as B YV to BY, or AO
-FOR to AO, that is (since CA and CO, CO and CR, and by division AO
and OR are proportional), as CA + CO to CA, or, if BY be bisected in E,
as 2CE to CB. Therefore (by Cor. 1, Prop. XLIX), the length of the
rectilinear part of the thread PT is always equal to the arc of the cycloid
PS, and the whole thread APT is always equal to the half of the cycloid
APS, that is (by Cor. 2, Prop. XLIX), to the length AR. And there¬
fore contrariwise, if the string remain always equal to the length AR, the
point T will always move in the given cycloid QRS. Q.E.D.

Cor. The string AR is equal to the semi-cycloid AS, and therefore has
the same ratio to AC the semi-diameter of the exterior globe as the like
semi-cycloid SR has to CO the semi-diameter of the interior globe.

PROPOSITION LI. THEOREM XVIII.

If a centripetal force tending on all sides to the centre C of a globe, be in
all places as the distance of the place from the centre , and by this force
alone acting upon it, the body T oscillate {in the manner above de¬
scribed) in the perimeter of the cycloid QRS; I say, that all the oscil¬
lations, how unequal soever in themselves, will be performed in equal
times.

For upon the tangent TW infinitely produced let fall the perpendicular
CX, and join CT. Because the centripetal force with which the body T
is impelled towards C is as the distance CT, let this (by Cor. 2, of the
I iaws) be resolved into the parts CX, TX, of which CX impelling the
body directly from P stretches the thread PT, and by the resistance the
'hrcad makes to it is totally employed, producing no other effect; but the
other part TX, impelling the body transversely or towards X, directly
accelerates the motion in the cycloid. Then it is plain that the accelera-
/ tion of the body, proportional to this accelerating force, will be every

188

THE MATHEMATICAL PRINCIPLES

[Book 1

moment as the length TX, that is (because CV\
WV, and TX, TW proportional to them are given),
as the length TW, that is (by Cor. 1, Prop. XLIX)
as the length of the arc of the cycloid TR. If there¬
fore two pendulums APT, A/tf, be unequally drawn
aside from the perpendicular AR, and let fall together,
their accelerations will be always as the arcs to be de¬
scribed TR, £R. But the parts described at the
beginning of the motion are as the accelerations, thai
is, as the wholes that are to be described at the be-

described, and the subsequent accelerations proportional to those parts, are
also as the wholes, and so on. Therefore the accelerations, and consequently
the velocities generated, and the parts described with those velocities, and
the parts to be described, are always as the wholes ; and therefore the parts
to be described preserving a given ratio to each other will vanish together,
that is, the two bodies oscillating will arrive together at the perpendicular AR.
And since on the other hand the ascent of thependulums from the lowest place
R through the same cycloidal arcs with a retrograde motion, is retarded in
the several places they pass through by the same forces by which their de¬
scent was accelerated; it is plain that the velocities of their ascent and de¬
scent through the same arcs are equal, and consequently performed in equal
times ; and, therefore, since the two parts of the cycloid RS and RQ lying
on either side of the perpendicular are similar and equal, the two pendu¬
lums will perform as well the wholes as the halves of their oscillations in
the same times. Q.E.D.

' Cor. The force with which the body T is accelerated or retarded in any
place T of the cycloid, is to the whole weight of the same body in the
highest place S or Q as the arc of the cycloid TR is to the arc SR or QR

PROPOSITION LII. PROBLEM XXXIY.

To define the velocities of the pendulums in the several places , and the
times in which both the entire oscillations , and the several parts of them
are performed.

About any centre G, with the interval GH equal to
the arc of the cycloid RS, describe a semi-circle HKM
bisected by the semi-diameter GK. And if a centripe¬
tal force proportional to the distance of the places from
the centre tend to the centre G, and it be in the peri¬
meter HIK equal to the centripetal force in the perime¬
ter of the globe QOS tending towards its centre, and at
the same time that the pendulum T is let fall from the
highest place S, a body, as L, is let fall from H to G ; then because th«

Sec. X.J of natural philosophy. IS9

forces which act upon the bodies are equal at the be¬
ginning, and always proportional to the spaces to be
described TR, LG, and therefore if TR and LG are
equal, arc also equal in the places T and L, it is plain
that those bodies describe at the beginning equal spaces
ST, HL, and therefore are still acted upon equally, and continue to describe
equal spaces. Therefore by Prop. XXXVIII, the time in which the body
describes the arc ST is to the time of one oscillation, as the arc HI the time
in which the body H arrives at L, to the semi-periphery IIKM, the time
in which the body H will come to M. And the velocity of the pendulous
body in the place T is to its velocity in the lowest place R, that is, the
velocity of the body H in the place L to its velocity in the place G, or the
momentary increment of the line HL to the momentary increment of the
line HG (the arcs HI, HK increasing with an equable flux) as the ordinate
LI to the radius GK, or as v/SR 2 — Til 2 to SR. Hence, since in unequal
oscillations there are described in equal time arcs proportional to the en¬
tire arcs of the oscillations, there are obtained from the times given, both
the velocities and the arcs described in all the oscillations universally.
Which was first required.

Let now any pendulous bodies oscillate in different cycloids described
within different globes, whose absolute forces are also different; and if the
absolute force of any globe QOS be called V, the accelerative force with
which the pendulum is acted on in the circumference of this globe, when it
begins to move directly towards its centre, will be as the distance of the
pendulous body from that centre and the absolute force of the globe con-
junctly, that is, as CO X V. Therefore the lineola HY, which is as this
accelerated force CO X V, will be described in a given time; and if there
be erected the perpendicular YZ meeting the circumference in Z, the nascent
arc HZ will denote that given time. But that nascent arc HZ is in the
subduplicate ratio of the rectangle GHY, and therefore as v/GH X CO X V
Whence the time of an entire oscillation in the cycloid QRS (it being as
the semi-periphery HKM, which denotes that entire oscillation, directly ;
and as the arc HZ which in like manner denotes a given time inversely)
will be as GH directly and -/GH X CO X V inversely; that is, because

GH and SR are equal, as ^ QQ^ ' y ? or (by Lor. Prop. L,) as

Therefore the oscillations in all globes and cycloids, performed with what
absolute forces soever, are in a ratio compounded of the subduplicate ratio of
the length of the string directly, and the subduplicate ratio of the distance
between the point of suspension and the centre of the globe inversely, and
the subduplicate ratio of the absolute force of the globe inversely also
Q.R.I.

i90

THE MATHEMATICAL PRINCIPLES

[Bo^k 1.

Cor. 1. Hence also the times of oscillating, falling, and revolving bodies
may be compared among themselves. For if the diameter of the wheel
with which the cycloid is described within the globe is supposed equal to
the semi-diameter of the globe, the cycloid will become a right line passing
through the centre of the globe, and the oscillation will be changed into a
descent and subsequent ascent in that right line. Whence there is given
both the time of the descent from any place to the centre, and the time equal
to it in which the body revolving uniformly about the centre of the globe
at any distance describes an arc of a quadrant For this time (by
Case 2) is to the time of half the oscillation in any cycloid QRS as 1 to
AR
^ AC'

Cor. 2. Hence also follow 7 what Sir Christopher Wren and M. Huygens
have discovered concerning the vulgar cycloid. For if the diameter of the
globe be infinitely increased, its sphaerical superficies will be changed into a
plane, and the centripetal force will act uniformly in the direction of lines
perpendicular to that plane, and this cycloid of our’s will become the same
with the common cycloid. But in that case the length of the arc of the
cycloid between that plane and the describing point will become equal to
four times the versed sine of half the arc of the wheel between the same
plane and the describing point, as was discovered by Sir Christopher Wren.
And a pendulum between two such cycloids will oscillate in a similar and
equal cycloid in equal times, as M. Huygens demonstrated. The descent
of heavy bodies also in the time of one oscillation will be the same as M.
Huygens exhibited.

The propositions here demonstrated are adapted to the true constitution
of the Earth, in so far as wheels moving in any of its great circles will de¬
scribe, by the motions of nails fixed in their perimeters, cycloids without the
globe; and pendulums, in mines and deep caverns of the Earth, must oscil¬
late in cycloids within the globe, that those oscillations may be performed
in equal times. For gravity (as will be shewn in the third book) decreases
in its progress from the superficies of the Earth; upwards in a duplicate
ratio of the distances from the centre of the Earth; downwards in a sim¬
ple ratio of the 3ame.

PROPOSITION LIII. PROBLEM XXXV.

Granting the quadratures of curvilinear figures , it is required to find
the forces with which bodies moving in given curve lines may always
perform their oscillations in equal times.

Let the body T oscillate in any curve line STRQ,, whose axis is AR
passing through the centre of force C. Draw TX touching that curve in
any place of the body T, and in that tangent TX take TY equal to the
arc TR. The length of that arc is known from the common methods used

o

Sec. X.

OF NATURAL PHILOSOPHY.

191

for the quadratures of figures. From the point Y
draw the right line YZ perpendicular to the tangent.

Draw CT meeting that perpendicular in Z, and the
centripetal force will be proportional to the right line
TZ. Q.E.I.

For if the force with which the body is attracted
from T towards C be expressed by the right line TZ
taken proportional to it, that force will be resolved
into two forces TY, YZ, of which YZ drawing the
body in the direction of the length of the thread PT,
docs not at all change its motion; whereas the other
force TY directly accelerates or retards its mction in the curve STRQ
Wherefore since that force is as the space to be described TR, the acceler¬
ations or retardations of the body in describing two proportional parts ft*
greater and a less) of two oscillations, will be always as those parts, and
therefore will cause those parts to be described together. But bodies w 7 hich
continually describe together parts proportional to the wholes, will describe

the wholes together

also. Q.E.D.

Cor. 1. Hence if the body T, hanging by a rectilinear thread
AT from the centre A, describe the circular arc STRQ.,
and in the mean time be acted on by any force tending
downwards with parallel directions, which is to the uni¬
form force of gravity as the arc TR to its sine TN, the
times of the several oscillations will be equal. For because
TZ, AR are parallel, the triangles ATN, ZTY are similar; and there¬
fore TZ will be to AT as TY to TN; that is, if the uniform force of
gravity be expressed by the given length AT, the force TZ. by which the
oscillations become isochronous, will be to the force of gravity AT, as the
arc TR equal to TY is to TN the sine of that arc.

Cor. 2. And therefore in clocks, if forces were impressed by some ma¬
chine upon the pendulum which preserves the motion, and so compounded
with the force of gravity that the whole force tending downwards should
be always as a line produced by applying the rectangle under the arc TR
and the radius AR to the sine TN, all the oscillations will become
isochronous.

PROPOSITION IJY. PROBLEM XXXVI.

Granting the quadratures of curvilinear figures , it is required to find
the times in which bodies by means of any centripetal force will descend
or ascend in any curve lines described in a plane passing through the
centre of force.

Let the body descend from any place S, and move in any curve ST/R
given in a plane passing through the centre of force C. Join CS, and lei

192

THE MATHEMATICAL PRINCIPLES

[Book 1

it be divided into innumerable equal parts, and let
Del be one of those parts. From the centre C, with
the intervals CD, C d, let the circles DT, dt be de¬
scribed, meeting the curve line ST7R in T and t.
And because the law of centripetal force is given,
and also the altitude CS from which the body at
first fell, there will be given the velocity of the body
in any other altitude CT (by Prop. XXXIX). But
the time in whieh the body describes the lineola T/
is as the length of that lineola, that is, as the secant
of the angle /TC directly, and the velocity inversely.
Let the ordinate DN, proportional to this time, be made perpendicular to
the right line CS at the point D, and because D d is given, the rectangle
D d X DN, that is, the area DNwtf, will be proportional to the same time.
Therefore if PN??, be a curve line in which the point N is perpetually found,
and its asymptote be the right line SQ standing upon the line CS at right
angles, the area SQPND will be proportional to the time in whieh the body
in its descent hath described the line ST; and therefore that area bein'*
found, the time is also given. Q.E.I.

PROPOSITION LY. THEOREM XIX.

If a body move in any curve superficies , whose axis passes through the
centre of force , and from the body a perpendicular be let fall upon the
axis ; and a line parallel and equal thereto be drawn from any given
point of the axis ; I say , that this parallel line will describe an area
proportional to the time,

Let BKL be a curve superficies, T a body
revolving in it, STR a trajectory which the
body describes in the same, S the beginning
of the trajectory, OMK the axis of the curve
superficies, TN a right line let fall perpendic¬
ularly from the body to the axis; OP a line
parallel and equal thereto drawn from the
given point O in the axis; AP the orthogra¬
phic projection of the trajectory described by
the point P in the plane AOP in which the
revolving line OP is found; A the beginning
of that projection, answering to the point S;
TC a right line drawn from the body to the centre; TG a part thereof
proportional to the centripetal force with which the body tends towards the
centre C; TM a right line perpendicular to the curve superficies; TI a
part thereof proportional to the force of pressure with which the body urges

m:

\17~

N '

r

K

OF NATURAL PHILOSOPHY.

193

Sec. X.]

the superficies, and therefore with which it is again repelled by the super¬
ficies towards M; PTF a right line parallel to the axis and passing through
the body, and GF, IH right lines let fall perpendicularly from the points
G and I upon that parallel PHTF. I say, now, that the area AOP, de¬
scribed by the radius OP from the beginning of the motion, is proportional
to the time. For the force TG (by Cor. 2, of the Laws of Motion) is re¬
solved into the forces TF, FG; and the force TI into the forces TH, HI;
but the forces TF, 'TH, acting in the direction of the line PF perpendicular
to the plane AOP, introduce no change in the motion of the body but in a di¬
rection perpendicular to that plane. Therefore its motion, so far as it has
the same direction with the position of the plane, that is, the motion of the
point P, by which the projection AP of the trajectory is described in that
plane, is the same as if the forces TF, TH were taken away, and the body
were acted on by the forces FG, HI alone; that is, the same as ,f the body
were to describe in the plane AOP the curve AP by means of a centripetal
force tending to the centre O, and equal to the sum of the forces FG and
HI. But with such a force as that (by Prop. 1) the area AOP will be de¬
scribed proportional to the time. Q,.E.D.

Cor. By the same reasoning, if a body, acted on by forces tending to
two or more centres in any the same right line CO, should describe in a
free space any curve line ST, the area AOP would be always proportional
to the time.

PROPOSITION LVI. PROBLEM XXXVII.

Granting the quadratures of curvilinear figures , and szipposing that
there are given both the law of centripetal force tending to a given cen¬
tre , and the curve superficies ichose axis passes through that centre ;
it is required to find the trajectory which a body will describe in that
superficies , when going ojffrom a given place with a given velocity ,
and in a given direction in that superficies .

The last construction remaining, let the
body T go from the given place S, in the di¬
rection of a line given by position, and turn
into the trajectory sought STR, whose ortho¬
graphic projection in the plane BDO is AP.

And from the given velocity of the body in
the altitude SC, its velocity in any other al¬
titude TC will be also given. With that
velocity, in a given moment of time, let the
body describe the particle T£ of its trajectory,
and let P p be the projection of that particle
described in the plane AOP. Join Op, and
a little circle being described upon the curve superficies about the centre T

13

194 THE MATHEMATICAL PRINCIPLES [BOOR 1

with the interval T7 let the projection of that little circle in the plane AOP
be the ellipsis pQ. And because the magnitude of that little circle TV, and
TN or PO its distance from the axis CO is also given, the ellipsis pQ will
be given both in kind and magnitude, as also its position to the right line
PO. And since the area PO p is proportional to the time, and therefore
given because the time is given, the angle PO/? will be given. And thence
will be given p the common intersection of the ellipsis and. the right line
Op, together with the angle OP 'p, in which the projection APy? of the tra¬
jectory cuts the line OP. But from thence (by conferring Prop. XLI, with
its 2d Cor.) the manner of determining the curve AP/? easily appears.
Then from the several points P of that projection erecting to the plane
AOP, the perpendiculars PT meeting the curve superficies in T, there will
be o^iven the several points T of the trajectory. Q.E.I.

SECTION XI.

( f the motions of bodies tending to each other with centripetal forces .

I have hitherto been treating of the attractions of bodies towards an im¬
movable centre; though very probably there is no such thing existent in

nature. For attractions are made towards bodies, and the actions of the

f

bodies attracted and attracting are always reciprocal and equal, by Law III ;
so that if there are two bodies, neither the attracted nor the attracting body
is truly at rest, but both (by Cor. 4, of the Laws of Motion), being as it
were mutually attracted, revolve about a common centre of gravity. And
if there be more bodies, which are either attracted by one single one which
is attracted by them again, or which all of them, attract each other mutu¬
ally , these bodies will be so moved among themselves, as that their common
centre of gravity will either be at rest, or move uniformly forward in a
right line. I shall therefore at present go on to treat of the motion of
bodies mutually attracting each other; considering the centripetal forces
as attractions ; though perhaps in a physical strictness they may more truly
be called impulses. But these propositions are to be considered as purely
mathematical; and therefore, laying aside all physical considerations, I
make use of a familiar way of speaking, to make myself the more easily
understood by a mathematical reader.

PROPOSITION LVII. THEOREM XX.

Two bodies attracting each other mutually describe similar figures about
their common centre of gravity , and about each other mutually.

For the distances of the bodies from their common centre of gravity are
leciprocally as the bodies; and therefore in a given ratio to each other:
%nd thence, by composition of ratios, in a given ratio to the whole distance

Sec. XI. J

OF NATURAL PHILOSOPHY.

195

between thje bodies. Now these distances revolve about their common term
with an equable angular motion, because lying in the same right line they
never change their inclination to each other mutually But right lines
that are in a given ratio to each other, and revolve about their terms with
an equal angular motion, describe upon planes, which either rest with
those terms, or move with any motion not angular, figures entirely similar
round those terms. Therefore the figures described by the revolution of
these distances are similar. Q,.E.D.

PROPOSITION LYI1L. THEOREM XXL

If two bodies attract each other mutually with forces of any kind, and
in the mean time revolve about the common centre of gravity ; I say,
that , by the same forces, there may be described round either body un¬
moved a figure similar and equal to the figures which the bodies so
moving describe round each other mutually.

Let the bodies S and P revolve about their common centre of gravity
C, proceeding from S to T, and from P to Q,. Prom the given point s let.

v

there be continually drawn sp, sq, equal and parallel to SP, TQ,; and the
;ur vepqVj which the point p describes in its revolution round the immovable
point s, will be similar and equal to the curves which the bodies S and P’
describe about each other mutually; and therefore, by Theor. XX, similar
to the curves ST and PQ,V which the same bodies describe about their
common centre of gravity C.; and that because the proportions of the lines
SC, CP, and SP or sp, to each other, are given.

Case 1. The common centre of gravity C (by Cor. 4, of the Laws of Mo¬
tion) is either at rest, or moves uniformly in a right line. Let us first
suppose it at rest, and in s and p let there be placed two bodies, one im¬
movable in s, the other movable in p, similar and equal to the bodies S and
P. Then let the right lines PR and pr touch the curves PQ, and pq ki P
and p , and produce CQ and sq to R and r. And because the figures
CPRQ, sprq are similar, RQ will be to rq as CP to sp , and therefore in a
given ratio. Hence if the force with which the body P is attracted to¬
wards the body S, and by consequence towards the intermediate point the
centre C, were to the force with which the body p is attracted towards the
centre s. in the same given ratio, these forces would in equal times attract

196 THE MATHEMATICAL PRINCIPLES |BoOK 1

the bodies from the tangents PR,jor to the arcs PQ, pq, through the in¬
tervals proportional to them RQ, rq ; and therefore this last force (tending
to s ) would make the body p revolve in the curve pqv, which would becomf
similar to the curve PQV, in which the first force obliges the body P t(
revolve; and their revolutions would be completed in the same times
But because those forces are not to each other in the ratio of CP to sp, bu;
(by reason of the similarity and equality of the bodies S and s, P and p
and the equality of the distances SP, sp) mutually equal, the bodies h
equal times will be equally drawn from the tangents; and therefore th.V
the body p may be attracted through the greater interval rq, there is re¬
quired a greater time, which will be in the subduplicate ratio of the inter¬
vals ; because, by Lemma X, the spaces described at the very beginning ol
the motion are in a duplicate ratio of the times. Suppose, then the velocity
of the body p to be to the velocity of the body P in a subduplicate ratio of
the distance sp to the distance CP, so that the arcs pq , PQ,, which are in a
simple proportion to each other, may be described in times that are in n
subduplicate ratio of the distances ; and the bodies P, p, always attracted
by equal forces, will describe round the quiescent centres C and 5 similar
figures PQV, pqv , the latter of which pqv is similar and equal to the figure
which the body P describes round the movable body S. Q.E.D

Case 2. Suppose now that the common centre of gravity, together with
the space in which the bodies are moved among themselves, proceeds uni¬
formly in a right line ; and (by Cor. 6, of the Laws of Motion) all the mo¬
tions in this space will be performed in the same manner as before; and
therefore the bodies will describe mutually about each other the same fig¬
ures as before, which will be therefore similar and equal to the figure pqv.
Q.E.D.

Cor. 1. Hence two bodies attracting each other with forces proportional
to their distance, describe (by Prop. X) both round their common centre of
gravity, and round each other mutually concentrical ellipses; and, vice
versa, if such figures are described, the forces are proportional to the dis¬
tances.

Cor. 2. And two bodies, whose forces are reciprocally proportional to
the square of their distance, describe (by Prop. XI, XII, XIII), both round
their common centre of gravity, and round each other mutually, conic sec¬
tions having their focus in the centre about which the figures are described.
And, vice versa, if such figures are described, the centripetal forces are re¬
ciprocally proportional to the squares of the distance.

Cor. 3. Any two bodies revolving round their common centre of gravity
describe areas proportional to the times, by radii drawn both to that centre
and to each other mutually.

Sec. XL]

OP" NATURAL PHILOSOPHY.

197

PROPOSITION LIX. THRO REM XXII.

The periodic time of two bodies S and P revolving round their common
centre of gravity C,is to the periodic time of one of the bodies P re¬
volving round the other S remaining unmoved , and describing a fig¬
ure similar and equal to those ivhich the bodies describe about each
other mutually , in a subduplicate ratio of the other body S to the sum
of the bodies S + P.

For, by the demonstration of the last Proposition, the times in which
any similar arcs PQ, and pq are described are in a subduplicate ratio of the
distances CP and SP, or sp, that is, in a subduplicate ratio of the ody S
to the sum of the bodies S + P. And by composition of ratios, the sums
of the times in which all the similar arcs PQ and pq are described, that is,
the whole times in which the whole similar figures are described are in the
same subduplicate ratio. Q.E.D.

PROPOSITION LX. THEOREM XXIII.

If tivo bodies S and P, attracting each other with forces reciprocally pro¬
portional to the squares of their distance , revolve about their common
centre of gravity ; I say , that the principal axis of the ellipsis which
either of the bodies , as P, describes by this motion about the other S,
will be to the principal axis of the ellipsis , ivhich the same body P may
describe in the same periodical time about the other body S quiescent ,
as the sum of the ttvo bodies S + P to the first of two m,ean propor¬
tionals betiveen that sum and the other body S.

For if the ellipses described were equal to each other, their periodic times
by the last Theorem would be in a subduplicate ratio of the body S to the
sum of the bodies S 4- P. Let the periodic time in the latter ellipsis be
diminished in that ratio, and the periodic times will become equal; but,
by Prop. XV, the principal axis of the ellipsis will be diminished in a ratio
sesquiplicate to the former ratio; that is, in a ratio to which the ratio of
S to S 4* P is triplicate; and therefore that axis will be to the principal
axis of the other ellipsis as the first of two mean proportionals between S
4- P and S to S + P. And inversely the principal axis of the ellipsis de¬
scribed about the movable body will be to the principal axis of that described
round the immovable as S + P to the first of two mean proportionals be¬
tween S -f- P and S. Q.E.D.

PROPOSITION LXI. THEOREM XXIV.

If two bodies attracting each other with any kind of forces , and not
otherwise agitated or obstructed , are moved in any manner ivhatsoever,
those motions will be the same as if they did not at all attract each
other mutually } but were both attracted with the same forces by a third
body placed in their common centre of gravity ; and the law of the

19S

THE MATHEMATICAL PRINCIPLES

[Book L

attracting fortes will be the saw£ in respect of the distance of the.
bodies from the common centre , as in respect of the distance between
the two bodies.

For those forces with which the bodies attract each other mutually, by
tending to the bodies, tend also to the common centre of gravity lying di¬
rectly between them ; and therefore are the same as if they proceeded from
*an intermediate body. QJE.D.

And because there is given the ratio of the distance of either body from
that common centre to the distance between the two bodies, there is given,
-it course, the ratio of any power of one distance to the same power of the
.ther distance; and also the ratio of any quantity derived in any manner
from one of the distances compounded any how with given quantities, to
another quantity derived in like manner from the other distance, and as
many given quantities having that given ratio of the distances to the first
Therefore if the force with which one body is attracted by another be di¬
rectly or inversely as the distance of the bodies from each other, or a3 any
power of that distance; or, lastly, as any quantity derived after any man¬
ner from that distance compounded with given qnantities; then will the
same force with which the same body is attracted to the common centre of
gravity be in like manner directly or inversely as the distance of the at¬
tracted body from the common centre, or as any power of that distance ;
cr, lastly, as a quantity derived in like sort from that distance compounded
with analogous given quantities. That is, the law of attracting force will
be the same with respect to both distances. Q,.E.D.

PROPOSITION LXII. PROBLEM XXXVIII.

To determine the motions of two bodies which attract each other with
forces reciprocally proportional to the squares of the distance between
them , and are let fall from given places.

The bodies, by the last Theorem, will be moved in the same manner as
if they were attracted by a third placed in the common centre of their
gravity; and by the hypothesis that centre will be quiescent at the begin¬
ning of their motion, and therefore (by Cor. 4, of the Laws of Motion) will
be always quiescent. The motions of the bodies are therefore to be deter¬
mined (by Prob. XXV) in the same manner as if they were impelled by
forces tending to that centre; and then we shall have the motions of the
bodies attracting each other mutually. Q.E.I.

PROPOSITION LX III. PROBLEM XXXIX.

To determine the motions of two bodies attracting each other with forces
reciprocally proportional to the squares of their distance , and going
off f com given places in. given directions with given velocities.

The motions of the bodies at the beginning being given, there is given

OF NATURAL PHILOSOPHY.

1%

Sec. XL]

also the uniform motion of the common centre of gravity, and the motion
of the space which moves along with this centre uniformly in a right line,
and also the very first, or beginning motions of the bodies in respect of this
space. Then (by Cor. 5, of the Laws, and the last Theorem) the subse¬
quent motions will be performed in the same manner in that space, as if
that space together with the common centre of gravity were at rest, and as
if the bodies did not attract each other, but were attracted by a third body
placed in that centre. The motion therefore in this movable space of each"
body going off from a given place, in a given direction, with a given velo¬
city, and acted upon by a centripetal force tending to that centre, is to be
determined by Prob. IX and XXVI, and at the same time will be obtained
the motion of the other round the same centre. With this motion com¬
pound the uniform progressive motion of the entire system of the space and
the bodies revolving in it, and there will be obtained the absolute motion
of the bodies in immovable space. Q.E.I.

PROPOSITION LXIV. PROBLEM XL.

Supposing forces with which bodies mutually attract each other to in¬
crease in a simple ratio of their distances from the centres ; it is ro-
qnired to find the motions of several bodies among themselves.
Suppose the first two bodies T and L 3 ^

to have their common centre of gravity in 1 c
l). These, by Cor. 1, Theor. XXI, will S y
describe ellipses having their centres in D,
the magnitudes of which ellipses are
known by Prob. V.

Let now a third body S attract the two
former T and L with the accelerative forces ST, SL, and let it be attract¬
ed again by them. The force ST (by Cor. 2, of the Laws of Motion) is
resolved into the forces SD, DT; and the force SL into the forces SD and
DL. Now the forces DT, DL, which are as their sum TL, and therefore
as the accelerative forces with which the bodies T and L attract each other
mutually, added to the forces of the bodies T and L, the first to the first,
and the last to the last, compose forces proportional to the distances DT
and DL as before, but only greater than those former forces; and there¬
fore (by Cor. 1, Prop. X, and Cor. l,and S, Prop. IV) they will cause those
bodies to describe ellipses as before, but with a swifter motion. Tlie re¬
maining accelerative forces SD and DL, by the motive forces SD X T and
SD X L, which are as the bodies attracting those bodies equally and in the
direction of the lines TI, LK parallel to DS, do not at all change their situ¬
ations with respect to one another, but cause them equally to approach to
the line IK; which must be imagined drawn through the middle of the
body S, and perpendicular to the line DS. But that approach to the line

200

THE MATHEMATICAL PRINCIPLES

TBook I.

IK will be hindered by causing the system of the bodies T and L on one
side, and the body S on the other, with proper velocities, to revolve round
the common centre of gravity C. With such a motion the body S, because
the sum of the motive forces SD X T and SD X L is proportional to the
distance CS, tends to the centre C, will describe an ellipsis round the same,
centre C; and the point D, because the lines CS and CD are proportional,
will describe a like ellipsis over against it. But the bodies T and L, at¬
tracted by the motive forces SD X T and SD X L, the first by the first,
and the last by the last, equally and in the direction of the parallel lines TI
and LK, as was said before, will (by Cor. 5 and 6, of the Laws of Motion)
continue to describe their ellipses round the movable centre D, as before.
Q.E.I.

Let there be added a fourth body V, and, by the like reasoning, it will
be demonstrated that this body and the point C will describe ellipses about
the common centre of gravity B; the motions of the bodies T, L, and S
round the centres D and C remaining the same as before; but accelerated.
And by the same method one may add yet more bodies at pleasure. Q.E.I.

v This would be the case, though the bodies T and L attract each other
mutually with accelerative forces either greater or less than those with
which they attract the other bodies in proportion to their distance. Let
all the mutual accelerative attractions be to each other as the distances
multiplied into the attracting bodies ; and from what has gone before it
will easily be concluded that all the bodies will describe different ellipses
with equal periodical times about their common centre of gravity B, in an
immovable plane. Q.E.I.

PROPOSITION LXY. THEOREM XXV.

Bodies , whose forces decrease in a duplicate ratio of their distances from
their centres , may move among themselves in ellipses ; and by radii
drawn to the foci may describe areas proportional to the times very
nearly.

In the last Proposition we demonstrated that case in which the motions
will be performed exactly in ellipses. The more distant the law of the
forces is from the law in that case, the more will the bodies disturb each
others motions ; neither is it possible that bodies attracting each other
mutually according to the law supposed in this Proposition should move
exactly in ellipses, unless by keeping a certain proportion of distances from
each other. However, in the following crises the orbits will not much dif¬
fer from ellipses.

Case l. Imagine several lesser bodies to revolve about some very great
one at different distances from it, and suppose absolute forces tending to
t very one of the bodies proportional to each. And because (by Cor. 4, ot
the T aws) the common centre of gravity of them all is either at rest, oi

Src. XI.]

OF NATURAL PHILOSOPHY.

201

mores uniformly forward in a right line, suppose the lesser bodies so small
that the great body may be never at a sensible distance from that centre ;
and then the great body will, without any sensible error, be either at rest,
or move uniformly forward in a right line; and the lesser will revolve
about that great one in ellipses, and by radii drawn thereto will describe
areas proportional to the times; if we except the errors that may be intro¬
duced by the receding of the great body from the common centre of gravity,
or by the mutual actions of the lesser bodies upon each other. But the
lesser bodies may be so far diminished, as that this recess and the mutual
actions of the bodies on each other may become less than any assignable;
and therefore so as that the orbits may become ellipses, and the areas an¬
swer to the times, without any error that is not les3 than any assignable.
Q.E.O.

Case 2. Let us imagine a system of lesser bodies revolving about a very
great one in the manner just described, or any other system of two bodies
revolving about each other to be moving uniformly forward in a right line, and
in the mean time to be impelled sideways by the force of another vastly greater
body situate at a great distance. And because the equal accelerative forces
with which the bodies are impelled in parallel directions do not change the
situation of the bodies with respect to each other, but only oblige the whole
system to change its place while the parts still retain their motions among
themselves, it is manifest that no change in those motions of the attracted
bodies can arise from their attractions towards the greater, unless by the
inequality of the accelerative attractions, or by the inclinations of the lines
towards each other, in whose directions the attractions are made. Suppose,
therefore, all the accelerative attractions made towards the great body
to be among themselves as the squares of the distances reciprocally; and
then, by increasing the distance of the great body till the differences of fhe
right lines drawn from that to the others in respect of their length, and the
inclinations of those lines to each other, be less than any given, the mo¬
tions of the parts of the system will continue without errors that are not
less than any given. And because, by the small distance of those parts from
each other, the whole system is attracted as if it were but one body, it will
therefore be moved by this attraction as if it were one body ; that is, its
centre of gravity will describe about the great bod/ one of the conic sec¬
tions (that is, a parabola or hyperbola when the attraction is but languid
and an ellipsis when it is more vigorous); and by radii drawn thereto, it
will describe areas proportional to the times, without any errors but those
which arise from the distances of the parts, which are by the supposition
exceedingly small, and may be diminished at pleasure. Q.E.O.

By a like reasoning one may proceed to more compounded cases in in¬
finitum.

Cor 1 . In the second Case, the nearer the very great body approaches to

202

THE MATHEMATICAL PRINCIPLES

[Cook I

the system of two or more revolving bodies, the greater will the pertur¬
bation be of the motions of the parts of the system among themselves; be¬
cause the inclinations of the lines drawn from that great body to those
parts become greater ; and the inequality of the proportion is also greater.

Cor. 2. But the perturbation will be greatest of all, if we suppose the
accelerative attractions of the parts of the system towards the greatest body
of all are not to each other reciprocally as the squares of the distances
from that great body; especially if the inequality of this proportion be
greater than the inequality of the proportion of the distances from the
great body. For if the accelerative force, acting in parallel directions
and equally, causes no perturbation in the motions of the parts of the
system, it must of course, when it acts unequally, cause a perturbation some¬
where, which will be greater or less as the inequality is greater or less.
The excess of the greater impulses acting upon some bodies, and not acting
upon others, must necessarily change their situation among themselves. And
this perturbation, added to the perturbation arising from the inequality
and inclination of the lines, makes the whole perturbation greater.

Cor. El ence if the parts of this system move in ellipses or circles
without any remarkable perturbation, it is manifest that, if they are at all
impelled by accelerative forces tending to any other bodies, the impulse is
very weak, or else is impressed very near equally and in parallel directions
upon all of them.

PROPOSITION LXVI. THEOREM XXVI.

If three bodies whose forces decrease in a duplicate ratio of the distances
attract each other mutually ; and the accelerative attractions of any
two towards the third be between themselves reciprocally as the squares
of the distances ; and the two least revolve about the greatest; I say,
that the interior of the two revolving bodies will, by radii drawn to the
innermost and greatest, describe round that body areas more propor¬
tional to the t imes, and a figure more approaching to that of an ellip¬
sis having its focus in the point of concourse of the radii,if that great
body be agitated by those attractions, than it would do if that great
body were not attracted at all by the lesser, but remained at rest; or
than, it would if that great body were very much more or very much
less attracted, or very much more or very much less agitated, by the
attractions.

This appears plainly enough from the demonstration of the second
Corollary of the foregoing Proposition; but it may be made out after
this manner by a way of reasoning more distinct and more universally
convincing.

Case 1. Let the lesser bodies P and S revolve in the same plane about
the greatest body T, the body P describing the interior orbit PAB, and S

Sec. XI.J of natural philosophy. 203

the exterior orbit ESE. Let SK be the mean distance of the bodies P and
S; and let the accelerative attraction of the body P towards S, at that
mean distance, be expressed by that line SK. Make SL to SK as the

square of SK to the square of SP, and SL will be the accelerative attrac¬
tion of the body P towards S at any distance SP. Join PT, and draw
LM parallel to it meeting ST in M; and the attraction SL will be resolv¬
ed (by Cor. 2, of the Laws of Motion) into the attractions SM, LM. And
so the body P will be urged with a threefold accelerative force. One of
these forces tends towards T, and arises from the mutual attraction of the
bodies T and P. By this force alone the body P would describe round the
body T, by the radius PT, areas proportional to the times, and an
ellipsis whose focus is in the centre of the body T ; and this it would do
whether the body T remained unmoved, or whether it were agitated by that
attraction. This appears from Prop. XI, and Cor. 2 and 3 of Theor.
XXI. The other force is that of the attraction LM, which, because it
tends from P to T, will be superadded to and coincide with the former
force; and cause the areas to be still proportional to the times, by Cor. 3,
Theor. XXI. But because it is not reciprocally proportional to the square
of the distance PT, it will compose, when added to the former, a force
varying from that proportion ; which variation will be the greater by how
much the proportion of this force to the former is greater, cceteris paribus.
Therefore, since by Prop. XI, and by Cor. 2, Theor. XXI, the force with
which the ellipsis is described about the focus T ought to be directed to
that focus, and to be reciprocally proportional to the square of the distance
PT, that compounded force varying from that proportion will make the
orbit PAB vary from the figure of an ellipsis that has its focus in the point
T ; and so much the more by how much the variation from that proportion
is greater; and by consequence by how much the proportion of the second
force LM to the first force is greater, cceteris paribus. But now the third
force SM, attracting the body P in a direction parallel to ST, composes with
the other forces a new force which is no longer directed from P to T: and which
varies so much more from this direction by how much the proportion of this
third force to the other forces is greater, cceterisparibus ; and therefore causes
the body P to describe, by the radius TP, areas no longer proportional to the
times: and therefore makes the variation from that proportionality so much
greater by how much the proportion of this force to the others is greater.
But this third force will increase the variation of the orbit PAB from the

204

THE MATHEMATICAL PRINCIPLES

[Book I

elliptical figure before-mentioned upon two accounts; first because that
force is not directed from P to T ; and, secondly, because it is not recipro¬
cally proportional to the square of the distance PT. These things being
premised, it i3 manifest that the areas are then most nearly proportional to
the times, when that third force is the least possible, the rest preserving
their former quantity ; and that the orbit PAB does then approach nearest
to the elliptical figure above-mentioned, when both the second and third,
but especially the third force, is the least possible; the first force remain¬
ing in its former quantity.

Let the accelerative attraction of the body T towards S be expressed by
the line SN ; then if the accelerative attractions SM and SN were equal,
these, attracting the bodies T and P equally and in parallel directions
would not at all change their situation with respect to each other. The mo¬
tions of the bodies between themselves would be the same in that case as if
those attractions did not act at all, by Cor. 6, of the Laws of Motion. And,
by a like reasoning, if the attraction SN is less than the attraction SM, it
will take away out of the attraction SM the part SN, so that there will re¬
main only the part (of the attraction) MN to disturb the proportionality of
the areas and times, and the elliptical figure of the orbit. And in like
manner if the attraction SN be greater than the attraction SM, the pertur¬
bation of the orbit and proportion will be produced by the difference MN
alone. After this manner the attraction SN reduces always the attraction
SM to the attraction MN, the first and second attractions rema ning per¬
fectly unchanged; and therefore the areas and times come then nearest to
proportionality, and the orbit PAB to the above-mentioned elliptical figure,
when the attraction MN is either none, or the least that is possible; that
is, when the accelerative attractions of the bodies P and T approach as near
as possible to equality; that is, when the attraction SN is neither none at
all, nor less than the least of all the attractions SM, but is, as it were, a
mean between the greatest and least of all those attractions SM, that is
not much greater nor much less than the attraction SK. Q.E.D.

Case 2. Let now the lesser bodies P. S, revolve about a greater T in dif¬
ferent planes; and the force LM, acting in the direction of the line PT
situate in the plane of the orbit PAB, will have the same effect as before;
neither will it draw the body P from the plane of its orbit. But the other
force NM acting in the direction of a line parallel to ST (and which, there¬
fore, when the body S is without the line of the nodes is inclined to the
plane of the orbit PAB), besides the perturbation of the motion just now
spoken of as to longitude, introduces another perturbation also as to latitude,
attracting the body P out of the plane of its orbit. And this perturbation,
in any given situation of the bodies P and T to each other, will be as the
generating force MN; and therefore becomes least when the force MN ia
least, that is (as was just now shewn), where the attraction SN is not mucb
greater nor much less than the attraction SK. Q.E.D.

OF NATURAL PHILOSOPHY.

205

Sfc. XI.]

Cor. 1. Hence it may be easily collected, that if several less bodies P
8, R, (See., revolve about a very great body T, the motion of the innermost
revolving body P will be least disturbed by the attractions of the others,
when the great body is as well attracted and agitated by the rest (accord¬
ing to the ratio of the accelerative forces) as the rest are by each other
mutually.

Cor. 2. In a system of three bodies, T, P, S, if the accelerative attrac¬
tions of any two of them towards a third be to each other reciprocally as the
squares of the distances, the body P, by the radius PT, will describe its area
about the body T swifter near the conjunction A and the opposition B than it
will near the quadratures C and D. For every force with which the body P
is acted on and the body T is not, and which does not act in the direction of
the line PT, does either accelerate or retard the description of the area,
according as it is directed, whether in conseqventia or in antecedentia.
Such is the force NM. This force in the'passage of the body P frem C
to A is directed in consequentia- to its motion, and therefore accelerates
it; then as far as D in antecedentia, , and retards the motion; then in, con¬
sequentia as far as B ; and lastly in antecedentia as it moves from B to C.

Cor. 3. And from the same reasoning it appears that the body P cceteris
paribus , moves more swiftly in the conjunction and opposition than in the
quadratures.

Cor. 4. The orbit of the body P, cceteris paribus , is more curve at the
quadratures than at the conjunction and opposition. For the swifter
bodies move, the less they deflect from a rectilinear path. And besides the
force KL, or NM, at the conjunction and opposition, is contrary to the
force with which the body T attracts the body P, and therefore diminishes
that force; but the body P will deflect the less from a rectilinear path the
less it is impelled towards the body T.

Cor. 5. Hence the body P, cccteris paribus , goes farther from the body
T at the quadratures than at the conjunction and opposition. This is said,

however, supposing no regard had to the motion of eccentricity. For if
the orbit of the body P be eccentrical, its eccentricity (as will be shewn
presently by Cor. 9) will be greatest when the apsides are in the syzy-
gies; and thence it may sometimes come to pass that the body P, in ita
near approach to the farther apsis, may go farther from the body T at the
syzygies than at the quadratures.

Cor. 6 . Because the centripetal force of the central body T, by which

206

THE MATHEMATICAL PRINCIPLES

[Book 1

the body P is retained in its orbit, is increased at the quadratures by the
addition caused by the force LM, and diminished at the syzygie 3 by the
subduction caused by the force KL, and, because the force KL is greatei
than LM, it is more diminished than increased; and, moreover, since that
centripetal force (by Cor. 2, Prop. IV) is in a ratio compounded of the sim¬
ple ratio of the radius TP directly, and the duplicate ratio of the periodi¬
cal time inversely; it is plain that this compounded ratio is diminished by
the action of the force KL; and therefore that the periodical time, supposing
the radius of the orbit PT to remain the same, will be increased, and that
in the subduplicate of that ratio in which the centripetal force is diminish¬
ed ; and, therefore, supposing this radius increased or diminished, the peri¬
odical time will be increased more or diminished less than in the sesquipli-
cate ratio of this radius, by Cor. 6, Prop. IV. If that force of the central
body should gradually decay, the body P being less and less attracted would
go farther and farther from the centre T ; and, on the contrary, if it were
increased, it would draw nearer to it. Therefore if the action of the distant
body S, by which that force is diminished, were to increase and decrease
by turns, the radius TP will be also increased and diminshed by turns;
and the periodical time will be increased and diminished in a ratio com¬
pounded of the sesquiplicate ratio of the radius, and of the subduplicate oi
that ratio in which the centripetal force of the central body T is dimin¬
ished or increased, by the increase or decrease of the action of the distant
body S.

Cor. 7. It also follows, from what was before laid down, that the axis
of the ellipsis described by the body P, or the line of the apsides, does as
to its angular motion go forwards and backwards by turns, but more for¬
wards than backwards, and by the excess of its direct motion is in the
whole carried forwards. For the force with which the body P is urged to
the body T at the quadratures, where the force MN vanishes, is compound¬
ed of the force LM and the centripetal force with which the body T at¬
tracts the body P. The first force LM, if the distance PT be increased, is
increased in nearly the same proportion with that distance, and the other
force decreases in the duplicate ratio of the distance; and therefore the
sum of these two forces decreases in a less than the duplicate ratio of the
distance PT; and therefore, by Cor. 1, Prop. XLV, will make the line of
the apsides, or, which is the same thing, the upper apsis, to go backward.
But at the conjunction and opposition the force with which the body P is
urged towards the body T is the difference of the force KL, and of the
force with which the body T attracts the body P; and that difference, be¬
cause the force KL is very nearly increased in the ratio of the distance
PT, decreases in more-than the duplicate ratio of the distance PT; and
therefore, by' Cor. 1, Prop. XLV, causes the line of the apsides to go for¬
wards. In the places between the syzygies and the quadratures, the motion

OF NATURAL PHILOSOPHY.

20?

Sec. Xl.J

of the line of the apsides depends upon both of these causes conjunctly, so
that it either goes forwards or backwards in proportion to the excess ol
one of these causes above the other. Therefore since the force KL in the
syzygies is almost twice as great as the force LM in the quadratures, the
excess will be on the side of the force KL, and by consequence the line of
the apsides will be carried forwards. The truth of this and the foregoing

Corollary will be more easily understood by conceiving the system of the
two bodies T and P to be surrounded on every side by several bodies S,
S, S, &c., disposed about the orbit ESE. For by the actions of these bo¬
dies the action of the body T will be diminished on every side, and decrease
in more than a duplicate ratio of the distance.

Cor. 8. Put since the progress or regress of the apsides depends upon
the decrease of the centripetal force, that is, upon its being in a greater or
less ratio than the duplicate ratio of the distance TP, in the passage of
the body from the lower apsis to the upper; and upon a like increase in
its return to the lower apsis again ; and therefore becomes greatest where
the proportion of the force at the upper apsis to the force at the lower ap¬
sis recedes farthest from the duplicate ratio of the distances inversely; it
is plain, that, when the apsides are in the syzygies, they will, by reason of
the subducting force KL or NM — LM, go forward more swiftly ; and in
the quadratures by the additional force LM go backward more slowly.
Because the velocity of the progress or slowness of the regress is continued
for a long time; this inequality becomes exceedingly great.

Cor. 9. If a body is obliged, by a force reciprocally proportional to the
square of its distance from any centre, to revolve in an ellipsis round that
centre; and afterwards in its descent from the upper apsis to the lower
apsis, that force by a perpetual accession of new force is increased in more
than a duplicate ratio of the diminished distance ; it is manifest that the
body, being impelled always towards the centre by the perpetual accession
of this new force, will incline more towards that centre than if it were
urged by that force alone which decreases in a duplicate ratio of the di¬
minished distance, and therefore will describe an orbit interior to that
elliptical orbit, and at the lower apsis approaching nearer to the centre
than before. Therefore the orbit by the accession of this new force will
become more eccentrical. If now, while the body is returning from the
lower to the upper apsis, it should decrease by the same degrees by which
it increases before the body would return to its first distance; and there-

THE MATHEMATICAL PRINCIPLES [BOOK I.

fore if the force decreases in a yet greater ratio, the body, being now less
attracted than before, will ascend to a still greater distance, and so the ec¬
centricity of the orbit will be increased still more. Therefore if the ratio
of the increase and decrease of the centripetal force be augmented each
revolution, the eccentricity will be augmented also; and, on the contrary,
if that ratio decrease, it will be diminished.

Now, therefore, in the system of the bodies T, P, S, when the apsides of
the orbit PAB are in the quadratures, the ratio of that increase and de¬
crease is least of all, and becomes greatest when the apsides are in the
syzygies. If the apsides are placed in the quadratures, the ratio near the
apsides is less, and near the syzygies greater, than the duplicate ratio of the
distances ; and from that greater ratio arises a direct motion of the line of
the apsides, as was just now said. But if we consider the ratio of the
whole increase or decrease in the progress between the apsides, this is less
than the duplicate ratio of the distances. The force in the lower is to the
force in the upper apsis in less than a duplicate ratio of the distance of the
upper apsis from the focus of the ellipsis to the distance of the lower apsis
from the same focus; and, contrariwise, when the apsides are placed in the
syzygies, the force in the lower apsis is to the force in the upper apsis in a
greater than a duplicate ratio of the distances. For the forces LM in the
quadratures added to the forces of the body T compose forces in a less ra¬
tio ; and the forces KL in the syzygies subducted from the forces of the
body T, leave the forces in a greater ratio. Therefore the ratio of the
whole increase and decrease in the passage between the apsides is least at
the quadratures and greatest at the syzygies; and therefore in the passage
of the apsides from the quadratures to the syzygies it is continually aug¬
mented, and increases the eccentricity of the ellipsis; and in the passage
from the syzygies to the quadratures it is perpetually decreasing, and di
minishes the eccentricity.

Cor. 10. That we may give an account of the errors as to latitude, let
us suppose the plane of the orbit EST to remain immovable; and from
the cause of the errors above explained, it is manifest, that, of the two
forces NM, ML, which are the only and entire cause of them, the force
ML acting always in the plane of the orbit PAB never disturbs the mo¬
tions as to latitude; and that the force NM, when the nodes are in the
syzygies, acting also in the same plane of the orbit, does not at that time
affect those motions. But when the nodes are in the quadratures, it dis¬
turbs fhem very much, and, attracting the body P perpetually out of the
plane of its orbit, it diminishes the inclination of the plane in the passage
of the body from the quadratures to the syzygies, and again increases the
same in the passage from the syzygies to the quadratures. Hence it
comes to pass that when the body is in the syzygies, the inclination is
then least of all, and returns to the first magnitude nearly, when the body

OF NATURAL PHILOSOPHY.

209

Sec. XI.]

arrives at the next node. But if the nodes are situate at the octants after
the quadratures, that is, between C and A, B and B, it will appear, from

wnat was just now shewn, that in the passage of the body P from either
node to the ninetieth degree from thence, the inclination of the plane is
perpetually diminished; then, in the passage through the next 45 degrees
to the next quadrature, the inclination is increased; and afterwards, again,
in its passage through another 45 degrees to the next node, it is dimin¬
ished. Therefore the inclination is more diminished than increased, and
is therefore always less in the subsequent node than in the preceding one.
And, by a like reasoning, the inclination is more increased than diminish¬
ed when the nodes are in the other octants between A and D, B and C.
The inclination, therefore, is the greatest of all when the nodes are in the
syzygies In their passage from the syzygies to the quadratures the incli¬
nation is diminished at each appulse of the body to the nodes ; and be¬
comes least of all when the nodes are in the quadratures, and the body in
the syzygies ; then it increases by the same degrees by which it decreased
before; and, when the nodes come to the next syzygies, returns to its
former magnitude.

Cor. 11. Because when the nodes are in the quadratures the body P is
perpetually attracted from the plane of its orbit; and because this attrac¬
tion is made towards S in its passage from the node C through the con¬
junction A to the node D ; and to the contrary part in its passage from the
node D through the opposition B to the node C; it is manifest that, in its
motion from the node C, the body recedes continually from the former
plane CD of its orbit till it comes to the next node; and therefore at that
node, being now at its greatest distance from the first plane CD, it will
pass through the plane of the orbit EST not in D, the other node of that
plane, but in a point that lies nearer to the body S, which therefore be¬
comes a new place of the node in antecedentia to its former place. And,
by a like reasoning, the nodes will continue to recede in their passage
from this node to the next. The nodes, therefore, when situate in the
quadratures, recede perpetually; and at the syzygies, where no perturba¬
tion can be produced in the motion as to latitude, are quiescent: in the in¬
termediate places they partake of both conditions, and recede more slowly;
and, therefore, being always either retrograde or stationary, they will be
carried backwards, or in antecedentia , each revolution.

Cor. 12. All the errors described in these corrollaries arc a little greate?

14

210

THE MATHEMATICAL PRINCIPLES

Book L

at the conjunction of the bodies P, S, than at their opposition; because
the generating forces NM and ML are greater.

Cor. 13. And since the causes and proportions of the errors and varia¬
tions mentioned in these Corollaries do not depend upon the magnitude of
the body S, it follows that all things before demonstrated will happen, if
the magnitude of the body S be imagined so great as that the system of the
two bodies P and T may revolve about it. And from this increase of the
body S, and the consequent increase of its centripetal force, from which the
errors of the body P arise, it will follow that all these errors, at equal dis-
tances, will be greater in this case, than in the other where the body S re¬
volves about the system of the bodies P and T.

Cor. 14. But since the forces NM, ML, when the body S is exceedingly
distant, are very nearly as the force SK and the ratio PT to ST con-
junctly ; that is, if both the distance PT, and the absolute force of the body
S be given, as ST 3 reciprocally ; and since those forces NM, ML are the
causes of all the errors and effects treated of in the foregoing Corollaries;
it is manifest that all those effects, if the system of bodies T and P con¬
tinue as before, and only the distance ST and the absolute force of the body
S be changed, will be very nearly in a ratio compounded of the direct ratio
of the absolute force of the body S, and the triplicate inverse ratio of the
distance ST. Hence if the system of bodies T and P revolve about a dis¬
tant body S, those forces NM, ML, and their cifi ts, will be (by Cor. 2 and
6, Prop IV) reciprocally in a duplicate ratio of the periodical time. And
thence, also, if the magnitude of the bod} S be proportional to its absolute
force, those forces NM, ML, and their effects, will be directly as the cube
of the apparent diameter of the distant body S viewed from T, and so vice
versa. For these ratios are the same as the compounded ratio above men¬
tioned.

Cor. 15. And because if the orbits ESE and PAB, retaining their fig¬
ure, proportions, and inclination to each other, should alter their magni¬
tude; and the forces of the bodies S and T should either remain, or be
changed in any given ratio; these forces (that is, the force of the body T,
which obliges the body P to deflect from a rectilinear course into the orbit
PAB, and the force of the body S, which causes the body P to deviate from
that orbit) would act always in the same manner, and in the same propor¬
tion ; it follows, that all the effects will be similar and proportional, and
the times of those effects proportional also ; that is, that all the linear er¬
rors will be as tne diameters of the orbits, the angular errors the same as
before; and the times of similar linear errors, or equal angular errors, as
the periodical times of the orbits.

Cor. 16. Therefore if the figures of the orbits and their inclination to
each other be given, and the magnitudes, forces, and distances of the bodies
be any how changed, we may. from the errors and times of those errors in

Sec. XI.J

OF NATURAL PHILOSOPHY.

21!

one case, collect very nearly the errors and times of the errors in any other
case. But this may be done more expeditiously by the following method.
The forces NM, ML, other things remaining unaltered, are as the radius
TP; and their periodical effects (by Cor. 2, Lem. X) are as the forces and
the square of the periodical time of the body P conjunctly. These are the
linear errors of the body P ; and hence the angular errors as they appear
from the centre T (that is, the motion of the apsides and of the nodes, and all
the apparent errors as to longitude and latitude) are in each revolution of
the body P as the square of the time of the revolution, very nearly. Let
these ratios be compounded with the ratios in Cor. 14, and in any system
of bodies T, P, S, where P revolves about T very near to it, and T re¬
volves about S at a great distance, the angular errors of the body P, ob¬
served from the centre T, will be in each revolution of the body P as the
square of the periodical time of the body P directly, and the square of the
periodical time of the body T inversely. And therefore the mean motion
of the line of the apsides will be in a given ratio to the mean motion of
the nodes; and both those motions will be as the periodical time of the
body P directly, and the square of the periodical time of the body T in¬
versely. The increase or diminution of the eccentricity and inclination of
the orbit PAB makes no sensible variation in the motions of the apsides
and nodes, unless that increase or diminution be very great indeed.

Cor. 17. Since the line LM becomes sometimes greater and sometimes
less than the radius FT, let the mean quantity of the force LM be expressed

by that radius PT; and then that mean force will be to the mean force
SK or SN (which may be also expressed by ST) as the length PT to the
length ST. But the mean force SN or ST, by which the body T is re¬
tained in the orbit it describes about S, is to the force with which the body P
is retained in its orbit about T in a ratio compounded of the ratio of the
radius ST to the radius PT, and the duplicate ratio of the periodical time
of the body P about T to the periodical time of the body T about S. And,
ex (BqiiOj the mean force LM is to the force by which the body P is retain¬
ed in its orbit about T (or by which the same body P might revolve at the
distance PT in the same periodical time about any immovable point T) in
the same duplicate ratio of the periodical times. The periodical times
therefore being given, together with the distance PT, the mean force LM
is also given; and that force being given, there is given also the force MN,
very nearly, by the analogy of the lines PT and MN.

212

THE MATHEMATICAL PRINCIPLES

[Book I

Cok. IS. By the same laws by which the body P revolves about the
body T, let us suppose many fluid bodies to move round T at equal dis¬
tances from it; and to be so numerous, that they may all become contiguous
to each other, so as to form a fluid annulus, or ring, of a round figure, and
concentrical to the body T; and the several parts of this annulus, perform¬
ing their motions by the same law as the body P, will draw nearer to the
body T, and move swifter in the conjunction and opposition of themselves
and the body S, than in the quadratures. And the nodes of this annulus,
or its intersections with the plane of the orbit of the body S or T, will rest
at the syzygies ; but out of the syzygies they will be carried backward, or
in cintecedentia ; with the greatest swiftness in the quadratures, and more
slowly in other places. The inclination of this annulus also will vary, and
its axis will oscillate each revolution, and when the revolution is completed
will return to its former situation, except only that it will be carried round
a little by the precession of the nodes.

Cor. 19. Suppose now the spherical body T, consisting of some matter
not fluid, to be enlarged, and to extend its.*lf on every side as far as that
annulus, and that a channel were cut all round its circumference contain¬
ing water; and that this sphere revolves uniformly about its own axis in
the same periodical time. This water being accelerated and retarded by
turns (as in the last Corollary), will be swifter at the syzygies, and slower
at the quadratures, than the surface of the globe, and so will ebb and flow in
its channel after the manner of the sea. If the attraction of the body S were
taken away, the water would acquire no motion of flux and reflux by revolv-
.ng round the quiescent centre of the globe. The case is the same of a globe
moving uniformly forwards in a right line, and in the mean time revolving
about its centre (by Cor. 5 of the Laws of Motion), and of a globe uni¬
formly attracted from its rectilinear course (by Cor. 6, of the same Laws).
But let the body S come to act upon it, and by its unequable attraction the
uater will receive this new motion ; for there will be a stronger attraction
upon that part of the water that is nearest to the body, and a weaker upon
that part which is more remote. And the force LM will attract the w'ater
downwards at the quadratures, and depress it as far as the syzygies ; and the
force KL will attract it upwards in the syzygies, and withhold its descent,
and make it rise as far as the quadratures; except only in so far as the
motion of flux and reflux may be directed by the channel of the water, and
be a little retarded by friction.

Cor. 20. If, now, the annulus becomes hard, and the globe is diminished,
the motion of flux and reflux will cease ; but the oscillating motion of the
inclination and the praecession of the nodes will remain. Let the globe
have the same axis with the annulus, and perform its revolutions in the
same times, and at its surface touch the annulus within, and adhere to it;
then the globe partaking of the motion of the annulus, this whole compages

Sec. XI.

OF NATURAL PHILOSOPHY.

213

will oscillate, and the nodes will go backward, for the globe, as we shall
shew presently, is perfectly indifferent to the receiving of all impressions.
The greatest angle of the inclination of the annulus single is when the
nodes are in the syzygies. Thence in the progress of the nodes to the
quadratures, it endeavours to diminish its inclination, and by that endea¬
vour impresses a motion upon the whole globe. The globe retains this
motion impressed, till the annulus by a contrary endeavour destroys that
motion, and impresses a new motion in a contrary direction. And by this
means the greatest motion of the decreasing inclination happens when the
nodes are in the quadratures, and the least angle of inclination in the octants

after the quadratures; and, again, the greatest motion of roclination happens
when the nodes are in the syzygies; and the greatest angle of reclination in
the octants following. And the case is the same of a globe without this an¬
nulus, if it be a little higher or a little denser in the equatorial than in the
polar regions ; for the excess of that matter in the regions near the equator
supplies the place of the annulus. And though we should suppose ..the cen¬
tripetal force of this globe to be any how increased, so that all its parts
were to tend downwards, as the parts of our earth gravitate to the centre,
yet the phenomena of this and the preceding Corollary would scarce be al¬
tered ; except that the places of the greatest and least height of the water
will be different; for the water is now no longer sustained and kept in its
orbit by its centrifugal force, but by the channel in which it flows. And,
besides, the force LM attracts the water downwards most in the quadra¬
tures, and the force KL or NM — LM attracts it upwards most in the
syzygies. And these forces conjoined cease to attract the water downwards,
and begin to attract it upwards in the octants before the syzygies; and
cease to attract the water upwards, and begin to attract the water down¬
wards in the octants after the syzygies. And thence the greatest height of
the water may happen about the octants after the syzygies; and the least
height about the octants after the quadratures; excepting only so far as the
motion of ascent or descent impressed by these forces may by the vis insita
of the water continue a little longer, or be stopped a little sooner by impe¬
diments in its channel.

Cor. 21. For the same reason that redundant matter in the equatorial
regions of a globe causes the nodes to go backwards, and therefore by the
increase of that matter that retrogradation is increased, by the diminution
is diminished, and by the removal quite ceases: it follows, that, if more than

214

THE MATHEMATICAL PRINCIPLES

[Book 1

that redundant matter be taken away, that is, if the globe be either more
depressed, or of a more rare consistence near the equator than near the
poles, there will arise a motion of the nodes in consequentia.

Cor. 22. And thence from the motion of the nodes is known the consti¬
tution of the globe. That is, if the globe retains unalterably the same poles,
and the motion (of the nodes) be in antecedentia, there is a redundance oi
the matter near the equator; but if in consequentia, a deficiency. Sup¬
pose a uniform and exactly sphaerical globe to be first at rest in a free space ;
then by some impulse made obliquely upon its superficies to be driven from
its place, and to receive a motion partly circular and partly right forward.
Because this globe is perfectly indifferent to all the axes that passthrough
its centre, nor has a greater propensity to one axis or to one situation of
the axis than to any other, it is manifest that by its own force it will never
change its axis, or the inclination of it. Let now this globe be impelled
obliquely by a new impulse in the same part of its superficies as before.
and since the effect of an impulse is not at all changed by its coming sooner
or later, it is manifest that these tivo impulses, successively impressed, will
produce the same motion as if they were impressed at the same time; that,
is, the same motion as if the globe had been impelled by a simple force
compounded of them both (by Cor. 2, of the Laws), that is, a simple motion
about an axis of a given inclination. And the case is the same if the sec¬
ond impulse were made upon any other place of the equator of the first
motion ; and also if the first impulse were made upon any place in the
equator of the motion which would be generated by the second impulse
alone; and therefore, also, when both impulses are made in any places
whatsoever; for these impulses will generate the same circular motion as
if they were impressed together, and at once, in the place of the intersec¬
tions of the equators of those motions, which would be generated by each
of them separately. Therefore, a homogeneous and perfect globe will not
retain several distinct motions, but will unite all those that are impressed
on it, and reduce them into one; revolving, as far as in it lies, always witli
a simple and uniform motion about one single given axis, with an inclina¬
tion perpetually invariable. And the inclination of the axis, or the velocity
of the rotation, will not be changed by centripetal force. For if the globe
be supposed to be divided into two hemispheres, by any plane whatsoever
passing through its own centre, and the centre to which the force is direct¬
ed, that force will always urge each hemisphere equally ; and therefore will
not incline the globe any way as to its motion round its own axis. But
let there be added any where between the pole and the equator a heap oi
new matter like a mountain, and this, by its perpetual endeavour to recede
from the centre of its motion, will disturb the motion of the globe, and
cause its poles to wander about its superficies, describing circles about
themselves and their opposite points. Neither can this enormous evagatior

Sec. XI.] of natural philosophy. 2In

of the poles be corrected, unless by placing that mountain ei i er in one ol
the poles; in which case, by Cor. 21, the nodes of the equator will go for¬
wards ; or in the equatorial regions, in which case, by Cor. 20, the nodes
will go backwards; or, lastly, by adding on the other side of the axis anew
quantity of matter, by which the mountain may be balanced in its motion;
and then the nodes will either go forwards or backwards, as the mountain
and this newly added matter happen to be nearer to the pole or to the
equator.

PROPOSITION LXV1I. THEOREM XXVII.

The same laws of attraction being supposed , I say, that the exterior body
S does, by radii drawn to the point O, the common centre of gravity
of the interior bodies P aiid T, describe round that centre areas more
proportional to the times, and an orbit more approaching to the form
of an ellipsis having its focus in that cen 'v. than, it can describe
round the innermost and greatest body T by ra Hi drawn to that
body.

For the attractions of the body S towards T and
P compose its absolute attraction, which is more
directed towards O, the common centre of gravity s
of the bodies T and P, than it is to the s. reatest
body T ; and which is more in a reciprocal propor¬
tion to the square of the distance SO, than it is to the square of the distanci
ST ; as will easily appear by a little consideration.

PROPOSITION LXVIII. THEOREM XXVIII.

The same laws of attraction supposed , I say, that the exterior body S
will, by radii draion to O, the common centre of gravity of the interior
bodies P and T, describe round that centre areas more propor¬
tional to the times, and an orbit more approaching to the form of an
ellipsis having its focus in that centre, if the innermost and greatest
body be agitated by these attractions as well as the rest, than it would
do if that body were either at rest as not attracted, or were much more
or much less attracted, or much more or much less agitated.

This may be demonstrated after the same manner as Prop. LXVI, but
by a more prolix reasoning, which I therefore pass over. It will be suf¬
ficient to consider it after this manner. From the demonstration of the
last Proposition it is plain, that the centre, towards which the body S is
urged by the two forces conjunctly, is very near to the common centre of
gravity of those two other bodies. If this centre were to coincide with that
common centre, and moreover the common centre of gravity of all the three
bodies were at rest, the body S on one side, and the common centre of
gravity of the other two bodies on the other side, would describe true ellip-

216

THE MATHEMATICAL PRINCIPLES *

[Hook 1

ses about that quiescent common centre. This appears from Cor. 2, Pro])
LVIII, compared with what was demonstrated in Prop. LX1V, and LXV
Now this accurate elliptical motion will be disturbed a little by the dis¬
tance of the centre of the two bodies from the centre towards which the
third body S is attracted. Let there be added, moreover, a motion to the
common centre of the three, and the perturbation will be increased yet
more. Therefore the perturbation is least when the
common centre of the three bodies is at rest; that
is, when the innermost and greatest body T is at¬
tracted acccording to the same law as the rest are;
and is always greatest when the common centre of
the three, by the diminution of the motion of the body T, begins to be
moved, and is more and more agitated.

Cor. And hence if more lesser bodies revolve about the great one, it
may easily be inferred that the orbits described will approach nearer to
ellipses; and the descriptions of areas will be more nearly equable, if all
the bodies mutually attract and agitate each other with accelerative forces
that are as their absolute forces directly, and the squares of the distances
inversely ; and if the focus of each orbit be placed in the common centre
of gravity of all the interior bodies (that is, if the focus of the first and in¬
nermost orbit be placed in the centre of gravity of the greatest and inner¬
most body; the focus of the second orbit in the common centre of gravity
of the two innermost bodies; the focus of the third orbit in the common
centre of gravity of the three innermost; and so on), than if the innermost
body were at rest, and was made the common focus of all the orbits.

PROPOSITION LXIX. THEOREM XXIX.

[n a system of several bodies A, B, C, D, fyc., if any one of those bodies,
as A, attract all the rest , B, C, D, $*c.,with accelerative faxes that are
reciprocally as the squares of the distances from the attracting- body ;
amt another body, as B, attracts also the rest, A, C, D, &pc., with forces
that are reciprocally as the squares of the distances from the attract¬
ing body ; the absolute forces of the attracting bodies A and B will
be to each other as those very bodies A and B to 'which those forces
belong.

For the accelerative attractions of all the bodies B, C, D, towards A,
are by the supposition equal to each other at equal distances; and in like
manner the accelerative attractions of all the bodies towards B are also
equal to each other at equal distances. But the absolute attractive force
of the body A is to the absolute attractive force of the body B as the ac¬
celerative attraction of all the bodies towards A to the accelerative attrac¬
tion of all the bodies towards B at equal distances; and so is also the ac¬
celerative attraction of the body B towards A to the accelerative attraction

OF NATURAL PHILOSOPHY.

Sec. XI ]

21 7

of the body A towards B. But the accelerative attraction of the body B
towards A is to the accelerative attraction of the body A towards B as the
mass of the body A to the mass of the body B ; because the motive forces
which (by the 2d, 7th, and Sth Definition) are as the accelerative forces
and the bodies attracted conjunctly are here equal to one another by the
third Law. Therefore the absolute attractive force of the body A is to the
absolute attractive force of the body B a8 the mass of the body A to the
mass of the body B. Q.E.D.

Cor. 1. Therefore if each of the bodies of the system A, B, C, D, &c.
does singly attract all the rest with accelerative forces that are reciprocally
as the squares of the distances from the attracting body, the absolute forces
of all those bodies will be to each other as the bodies themselves.

Cor. 2. By a like reasoning, if each of the bodies of the system A, B,
C, D, &c., do singly attract all the rest with accelerative forces, which are
either reciprocally or directly in the ratio of any power whatever of the
distances from the attracting body; or which are defined by the distances
from each of the attracting bodies according to any common law ; it is plain
that the absolute forces of those bodies are as the bodies themselves.

Cor. 3. In a system of bodies whose forces decrease in the duplicate ra¬
tio of the distances, if the lesser revolve about one very great one in ellip¬
ses, having their common focus in the centre of that great body, and of a
figure exceedingly accurate; and moreover by radii drawn to that great
ody describe areas proportional to the times exactly ; the absolute forces
>{ those bodies to each other will be either accurately or very nearly in the
ratio of the bodies. And s > on the contrary. This appears from Cor. of
Prop.XLVIIl,compared with the first Corollary of this Prop.

SCHOLIUM.

These Propositions naturally lead us to the analogy there is between
centripetal forces, and the central bodies to which those forces used to be
directed; for it is reasonable to suppose that forces which are directed to
bodies should depend upon the nature and quantity of those bodies, as we
see they do in magnetical experiments. And when such cases occur, we
are to compute the attractions of the bodies by assigning to each of their
particles its proper force, and then collecting the sum of them all. I here
u^e the word attraction in general for any endeavour, of what kind soever,
made by bodies to approach to each other; whether that endeavour arise
from the action of the bodies themselves, as tending mutually to or agita¬
ting each other by spirits emitted; or whether it arises from the action
of the aether or of the air, or of any medium whatsoever* whether corporeal
or incorporeal, any how impelling bodies placed therein towards each other.
In the same general sense I use the word impulse, not defining in this trea¬
tise the species or physical qualities of forces, but investigating tbe quantities

218

THE MATHEAIATTPAr. PTlTYrTPI TTC

and mathematical proportions of them ; as I observed before in the Defi¬
nitions. In mathematics we are to investigate the quantities of forces
with their proportions consequent upon any conditions supposed ; then,
when we enter upon physics, we compare those proportions with the phe¬
nomena of Nature, that we may know what conditions of those forces an¬
swer to the several kinds of attractive bodies. And this preparation being
made, we argue more safely concerning the physical species, causes, and
proportions of the forces. Let us see, then, w r ith what forces sphaerical
bodies consisting of particles endued with attractive powers in the manner
above spoken of must act mutually upon one another : and what kind of
motions will follow from thence.

SECTION XII.

Of the attractive forces of sphcerical bodies.

PROPOSITION LXX. THEOREM XXX.

If to every point of a sphcerical surface there tend equal centripetal forces
decreasing in the duplicate ratio of the distances from those points ;
I say } that a corpuscle placed within that superficies will not be attract¬
ed by those forces any way.

Let HIKL, be that sphmrical superficies, and P a
corpuscle placed within. Through P let there be
drawn to this superficies to two lines I1K, IL, inter¬
cepting very small arcs HI, KL ; and because (by
Cor. 3, Lem. VII) the triangles HPI,LPK are alike,
those arcs will be proportional to the distances HP
LP; and any particles at HI and KL of the spheri¬
cal superficies, terminated by right lines passing through P, will be in the
duplicate ratio of those distances. Therefore the forces of these particles
exerted upon the body P are equal between themselves. For the forces arc
as the particles directly, and the squares of the distances inversely. And
these two ratios compose the ratio of equality. The attractions therefore,
being made equally towards contrary parts, destroy each other. And by a
like reasoning all the attractions through the whole sphmrical superficies
are destroyed by contrary attractions. Therefore the body P will not be
any way impelled by those attractions. Q.E.D.

PROPOSITION LXXI. THEOREM XXXI.

The same things supposed as above , I say, that a corpus!c placed with¬
out the sphcerical sitperfcies is attracted towards the centre of the
sphere with a force reciprocally proportional to the square of its dis¬
tance from that centre.

Let AHKB, ah kb, be two equal sphaerical superficies described about

OF NATURAL PHILOSOPHY.

219

Sec. XII.]

the centre S, s ; their diameters AB, ab ; and let P and p be two corpus¬
cles situate without the spheres in those diameters produced. Let there

be drawn from the corpuscles the lines PHK, PIL, phk, pil, cutting off
from the great circles AHB, ahb , the equal arcs HK, hk , IL ; il ; and to
those lines let fall the perpendiculars SD, sd, SE, sp, 1R, ir ; of which let
SD, sd, cut PL,joJ, in F and f. Let fall also to the diameters the perpen¬
diculars IQ, iq. Let now the angles DPE, dpe, vanish; and because DS
and ds, ES and es are equal, the lines PE, PF, and pe, pf, and the lineolae
DF, df may be taken for equal; because their last ratio, when the angles
DPE, dpe vanish together, is the ratio of equality. These things then
supposed, it will be, as PI to PF so is RI to DF, and as pf to pi so is df or
DF to ri ; and, ex cequo, as PI X pf to PF X pi so is RI to ri, that is
(by Cor. 3, Lem VII), so is the arc IH to the arc ih. Again, PI is to PS
as IQ to SE, and ps to pi as se or SE to iq ; and, ex cequo, PI X ps to
PS X pi as IQ. to iq. And compounding the ratios PI 2 X pf X ps is to
pi 2 X PF X PS, as IH X IQ to ih X iq ; that is, as the circular super¬
ficies which is described by the arc IH, as the semi-circle AKB revolves
about the diameter AB, is to the circular superficies described by the arc ih
as the semi-circle akb revolves about the diameter ab. And the forces
with which these superficies attract the corpuscles P and p in the direction
of lines tending to those superficies are by the hypothesis as the superficies
themselves directly, and the squares of the distances of the superficies from
those corpuscles inversely; that is, as pf X ps to PF XPS. And these
forces again are to the oblique parts of them which (by the resolution of
forces as in Cor. 2, of the Laws) tend to the centres in the directions of the
lines PS, Jos*, as PI to PQ, andy?i to pq ; that is (because of the like trian¬
gles PIQ and PSF, piq and psf), as PS to PF and ps to pf. Thence ex
cequo, the attraction of the corpuscle P towards S is to the attraction of

the corpuscle p towards 5 as ^ ^PS^ is to that is,

as ps 2 to PS 2 . And, by a like reasoning, the forces with which the su¬
perficies described by the revolution of the arcs KL, kl attract those cor¬
puscles, will be asjt?s 2 to PS 2 . And in the same ratio will be the forces
of all the circular superficies into which each of the sphaerical superficies
may be divided by taking sd always equal to SD, and se equal to SE. And
therefore, by composition, the forces of the entire sphaerical superficies ex¬
erted upon those corpuscles will be in the same ratio. Q.E.D

220

THE MATHEMATICAL PRINCIPLES

[Book 1

PROPOSITION LXXII. THEOREM XXXII.

If to the several points of a sphere there tend equal centripetal forces de¬
creasing in a duplicate ratio of the distances from those points ; and
there be given both the density of the sphere and the ratio of the di¬
ameter of the sphere to the distance of the corpuscle from its centre ;
I say, that the force with which the corpuscle is attracted is propor¬
tional to the semi-diameter of the sphere.

For conceive two corpuscles to be severally attracted by two spheres, one
by one, the other by the other, and their distances from the centres of the
spheres to be proportional to the diameters of the spheres respectively , and
the spheres to be resolved into like particles, disposed in a like situation
to the corpuscles. Then the attractions of one corpuscle towards the sev¬
eral particles of one sphere will be to the attractions of the other towards
as many analogous particles of the other sphere in a ratio compounded of
the ratio of the particles directly, and the duplicate ratio of the distances
inversely. But the particles are a3 the spheres, that is, in a triplicate ra¬
tio of the diameters, and the distances are as the diameters; and the first
ratio directly with the last ratio taken twice inversely, becomes the ratio
of diameter to diameter. Q,.E.D.

Cor. 1. Hence if corpuscles revolve in circles about spheres composed
of matter equally attracting, and the distances from the centres of the
spheres be proportional to their diameters, the periodic times will be equal.

Cor. 2. And, vice versa , if the periodic times are equal, the distances
will be proportional to the diameters. These two Corollaries appear from
Cor. 3, Prop. IV.

Cor. 3. If to the several points of an^ two solids whatever, of like fig¬
ure and equal density, there tend equal centripetal forces decreasing in a
duplicate ratio of the distances from those points, the forces, with which
corpuscles placed in a like situation to those two solids will be attracted
by them, will be to each other as the diameters of the solids.

PROPOSITION LXXIII. THEOREM XXXH1.

If to the several points of a given sphere there tend equal centripetal forces
decreasing in a duplicate ratio of the distances from the pomts ; 1
say, that a corpuscle placed 'within the sphere is attracted by a force
proportional to its distance from the centre.

In the sphere ABCD, described about the centre S,
let there be placed the corpuscle P; and about the
same centre S, with the interval SP, conceive de-
B scribed an interior sphere PEQ,F. It is plain (by
Prop. LXX) that the concentric sphmrical superficies,
of which the difference AEBF of the spheres is com¬
posed, have no effect at all upon the body P, their at-

OF NATURAL PHILOSOPHY.

Sec. XII.]

221

tractions being destroyed by contrary attractions. There remains, there¬
fore, only the attraction of the interior sphere PEQF. And (by Prop.
LX XII) this is as the distance PS. Q.E.D.

SCHOLIUM.

By the superficies of which I here imagine the solids composed, I do not
mean superficies purely mathematical, but orbs so extremely thin, that
their thickness is as nothing; that is, the evanescent orbs of which the sphere
will at last consist, when the number of the orbs is increased, and their
thickness diminished without end. In like manner, by the points of which
lines, surfaces, and solids are said to be composed, are to be understood
equal particles, whose magnitude is perfectly inconsiderable.

PROPOSITION LXXIV. THEOREM XXXIV.

The same things supposed , I say , that a corpuscle situate without the
sphere is attracted with a force reciprocally proportional to the square
of its distance from the centre .

For suppose the sphere to be divided into innumerable concentric sphe¬
rical superficies, and the attractions of the corpuscle arising from the sev¬
eral superficies will be reciprocally proportional to the square of the dis¬
tance of the corpuscle from the centre of the sphere (by Prop. LXXI).
And, by composition, the sum of those attractions, that is, the attraction
of the corpuscle towards the entire sphere, will be in the same ratio. Q.E.D.

Cor. 1. Hence the attractions of homogeneous spheres at equal distances
from the centres will be as the spheres themselves. For (by Prop. LXXII)
if the distances be proportional to the diameters of the spheres, the forces
will be as the diameters. Let the greater distance be diminished in that
ratio; and the distances now being equal, the attraction will be increased
in the duplicate of that ratio; and therefore will be to the other attraction
in the triplicate of that ratio ; that is, in the ratio of the spheres.

Cor. 2. At any distances whatever the attractions are as the spheres
applied to the squares of the distances.

Cor. 3. If a corpuscle placed without an homogeneous sphere is attract¬
ed by a force reciprocally proportional to the square of its distance from
the centre, and the sphere consists of attractive particles, the force of every
particle will decrease in a duplicate ratio of the distance from each particle.

PROPOSITION I,XXV. THEOREM XXXV.

If to the several points of a given sphere there tend equal centripetal forces
decreasing in a duplicate ratio of the distances from the points ; Isay ,
that another similar sphere will be attracted by it with a force recip¬
rocally proportional to the square of the distance of the centres.

For the attraction of every particle is reciprocally as the square of its

222

•THE MATHEMATICAL PRINCIPLES

|Book L

distance from the centre of the attracting sphere (by Prop. LXXIV), and
is therefore the same as if that whole attracting force issued from one sin¬
gle corpuscle placed in the centre of this sphere. But this attraction is as
great as on the other hand the attraction of the same corpuscle would be,
if that were itself attracted by the several particles of the attracted sphere
with the same force with which they are attracted by it. But that attrac¬
tion of the corpuscle would be (by Prop. LXXIV) reciprocally propor¬
tional to the square of its distance from the centre of the sphere ; therefore
the attraction of the sphere, equal thereto, is also in the same ratio. Q,.E. D.

Cor. 1. The attractions of spheres towards other homogeneous spheres
are as the attracting spheres applied to the squares of the distances of their
centres from the centres of those which they attract.

Cor. 2. The case is the same when the attracted sphere does also at¬
tract. For the several points of the one attract the several points of the
other with the same force with which they themselves are attracted by the
others again; and therefore since in all attractions (by Law III) the at¬
tracted and attracting point are both equally acted on, the force will be
doubled by their mutual attractions, the proportions remaining.

Cor. 3. Those several truths demonstrated above concerning the motion
of bodies about the focus of the conic sections will take place when an
attracting sphere is placed in the focus, and the bodies move without the
sphere.

Cor. 4. Those things which were demonstrated before of the motion of
bodies about the centre of the conic sections take place when the motions
are performed within the sphere.

PROPOSITION LXXVI. THEOREM XXXVI.

If spheres be however dissimilar (as to density of matter and attractive
force) in the same ratio onward from the centre to the circumference ;
but every where similar, at every given distance from the centre, on all
sides round about; and the attractive force of every point decreases
in the duplicate ratio of the distance of the body attracted ; Isay,
that the whole force with which one of these spheres attracts the other
will be reciprocally proportional to the square of the distance of the
centres .

Imagine several concentric similar
spheres, AB, CD, EF, &c.. the inner¬
most of which added to the outermost
may compose a matter more dense to¬
wards the centre, or subducted from
them may leave the same more lax and
rare. Then, by Prop. LXXV, these
spheres will attract other similar con-

Sec. XJL]

OF NATURAL PHILOSOPHY.

223

sentric spheres GH, IK, LM, &c., each the other, with forces reciprocally
proportional to the square of the distance SP. And, by composition or
division, the sum of all those forces, or the excess of any of them above
the others; that is, the entire force with which the whole sphere AB (com¬
posed of any concentric spheres or of their differences) will attract the
whole sphere GH (composed of any concentric spheres or their differences)
in the same ratio. Let the number of the concentric spheres be increased
in infinitum, so that the density of the matter together with the attractive
force may, in the progress from the circumference to the centre, increase or
decrease according to any given law ; and by the addition of matter not at¬
tractive, let the deficient density be supplied, that so the spheres may acquire
any form desired; and the force with which one of these attracts the other
will be still, by the former reasoning, in the same ratio of the square of the
distance inversely. QJE.l).

Cor. 1. Hence if many spheres of this kind, similar in all respects, at¬
tract each other mutually, the accelerative attractions of each to each, at
any equal distances of the centres, will be as the attracting spheres.

Cor. 2. And at any unequal distances, as the attracting spheres applied
to the squares of the distances between the centres.

Cor. 3. The motive attractions, or the weights of the spheres towards
one another, will be at equal distances of the centres as the attracting and
attracted spheres conjunctly ; that is, as the products arising from multi¬
plying the spheres into each other.

Cor. 4. And at unequal distances, as those products directly, and the
squares of the d'- stances between the centres inversely.

Cor. 5. These proportions take place also when the attraction arises
from the attractive virtue of both spheres mutually exerted upon each
other. For the attraction is only doubled by the conjunction of the forces,
the proportions remaining as before.

Cor. 6. If spheres of this kind revolve about others at rest, each about
each ; and the distances between the centres of the quiescent and revolving
bodies are proportional to the diameters of the quiescent bodies ; the peri¬
odic times will be equal.

Cor. 7. And, again, if the periodic times are equal, the distances will
be proportional to the diameters.

Cor. 8. All those truths above demonstrated, relating to the motions
of bodies about the foci of conic sections, will take place when an attract¬
ing sphere, of any form and condition like that above described, is placed
in the focus.

Cor. 9. And also when the revolving bodies are also attracting spheres
of any condition like that above described.

224

THE MATHEMATICAL PRINCIPLES

[Book I.

PROPOSITION LXXVIl. THEOREM XXXVII.

Tf to the several points of spheres there tend centripetal forces propor¬
tional to the distances of the points from the attracted bodies ; I say,
that the compounded force with which two spheres attract each other
mutually is as the distance between the centres of the spheres.

Case 1. Let AEBP be a sphere; S its
centre. P a corpuscle attracted: PA SB
the axis of the sphere passing through the
centre of the corpuscle ; EF, ef two planes
cutting the sphere, and perpendicular to
the axis, and equi-distant, one on one side,
the other on the other, from the centre of
the sphere; G and g the intersections of
the planes and the axis ; and H any point in the plane EF. The centri¬
petal force of the point PI upon the corpuscle P, exerted in the direction of
the line PH, is as the distance PH; and (by Cor. 2, of the Laws) the same
exerted in the direction of the line PG, or towards the. centre S, is as the
length PG. Therefore the force of all the points in the plane EF (that is,
of that whole plane) by which the corpuscle P is attracted towards the
centre S is as the distance PG multiplied by the number of those points,
that is, as the solid contained under that plane EF and the distance PG.
And in like manner the force of the plane ef, by which the corpuscle P is
attracted towards the centre S, is as that plane drawn into its distance P^,
or as the equal plane EF drawn into that distance P^ ; and the sum of the
forces of both planes as the plane EF drawn into the sum of the distances
PG -f- Pg-, that is, as that plane drawn into twice the distance PS of the
centre and the corpuscle ; that is, as twice the plane EF drawn into the dis¬
tance PS, or as the sum of the equal planes EF + ef drawn into the same
distance. And, by a like reasoning, the forces of all the planes in the
whole sphere, equi-distant on each side from the centre of the sphere, are
as the sum of those planes drawn into the distance PS, that is, as the
whole sphere and the distance PS conjunctly. Q.E.D.

Case 2. Let now the corpuscle P attract the sphere AEBF. And, by
the same reasoning, it will appear that the force with which the sphere is
attracted is as the distance PS. Q,.E.D.

Case 3. Imagine another sphere composed of innumerable corpuscles P:
and because the force with which every corpuscle is attracted is as the dis¬
tance of the oorpuscle from the centre of the first sphere, and as the same
sphere conjunctly, and is therefore the same as if it all proceeded from a
single corpuscle situate in the centre of the sphere, the entire force with
which all the corpuscles in the second sphere are attracted, that is, with
which that whole sphere is attracted, will be the same as if that sphere

Sec. X1I.J op natural philosophy. 225

were attracted by a force issuing from, a single corpuscle in the centre of
the first sphere; and is therefore proportional to the distance between the
centres of the spheres. Q.E.D.

Case 4. Let the spheres attract each other mutually, and the force will
be doubled, but the proportion will remain. Q.E.D.

Case 5. Let the corpuscle p be placed within
the sphere AEBF; and because the force of the
plane ef upon the corpuscle is as the solid contain¬
ed under that plane, and the distance^; and the B
contrary force of the plane EF as the solid con¬
tained under that plane and the distance pG ; the
force compounded of both will be as the difference
of the solids, that is, as the sum of the equal planes drawn into half the
difference of the distances ; that is, as that sum drawn into pS, the distance
of the corpuscle from the centre of the sphere. And, by a like reasoning,
the attraction of all the planes EF, ef throughout the whole sphere, that
is, the attraction of the whole sphere, is conjunctly as the sum of all the
planes, or as the whole sphere, and as jdS, the distance of the corpuscle from
the centre of the sphere. Q.E.D.

Case 6. And if there be composed a new sphere out of innumerable cor¬
puscles such as p, situate within the first sphere AEBF, it may be proved,
as before, that the attraction, whether single of one sphere towards the
other, or mutual of both towards each other, will be as the distance p S of
the centres. Q E.D.

PROPOSITION LXXVIII. THEOREM XXXVIII.

If spheres in the progress from the centre to the circumference be howcier
dissimilar and unequable, but similar on every side round about at all
given distances from the centre ; and the attractive force of every
point be as the distance of the attracted body ; I say, that the entire
force with 'which two spheres of this kind attract each other mutually
is proportional to the distance betioeen the centres of the spheres.

This is demonstrated from the foregoing Proposition, in the same man¬
ner as Proposition LXXVI was demonstrated from Proposition LXXV.

Cor. Those things that were above demonstrated in Prop. X and LXIV,
of the motion of bodies round the centres of conic sections, take place when
all the attractions are made by the force of sphaerical bodies of the condi¬
tion above described, and the attracted bodies are spheres of the same kind.

SCHOLIUM.

I have now explained the two principal cases of attractions; to wit,
when the centripetal forces decrease in a duplicate ratio of the distances!
'.r increase in a simple ratio of the distances, causing the bodies in botli

15

226

THE MATHEMATICAL PRINCIPLES

[Book I

cases to revolve in conic sections, and composing sphaerical bodies whose
centripetal forces observe the same law of increase or decrease in the recess
from the centre as the forces of the particles themselves do ; which is very
remarkable. It would be tedious to run over the other cases, whose con¬
clusions are less elegant and important, so particularly as I have dohe
these. I choose rather to comprehend and determine them all by one gen¬
eral method as follows.

LEMMA XXIX.

ff about the centre S there be described any circle as AEB, and about the
centre P there be also described two circles EP, ef, cutting the first in
E and e, and the line PS in P and f; and there be let fall to PS the
perpendiculars ED, ed; I say, that if the distance of the arcs EF, ef
be supposed to be infinitely diminished, the last ratio of the evanscent
line Dd to the evanescent line Pf is the same as that of the line PE to
the line PS.

For if the line Pe cut the arc EF in q ; and the right line Ee, which

coincides with the evanescent arc Ee, be produced, and meet the right line
PS in T ; and there be let fall from S to PE the perpendicular SG ; then,
because of the like triangles DTE, dTe, DES, it will be as D d to Ee so
DT to TE, or DE to ES ; and because the triangles, Yeq, ESG (by Lem.
VIII, and Cor. 3, Lem. VII) are similar, it will be as Ee to eq or F/ so ES
to SG ; and, ex aequo. , as T)d to Yf so DE to SG ; that is (because of the
similar triangles PDE, PGS), so is PE to PS. Q.E.D.

PROPOSITION LXXIX. THEOREM XXXIX.

Suppose a superficies as EFfe to have its breadth infinitely diminished ,
and to be just vanishing ; and that the same superficies by its revolu¬
tion round the axis PS describes a sphcerical concavo-convex solid, to
the severed equal particles of which there tend equal centripetal forces ;
I say, that the force with which that solid attracts a corpuscle situate
in P is in a ratio compounded of the ratio of the solid DE 2 X Ff and
the ratio of the force with which the given particle in the place Ff
would, attract the same corpuscle .

For if we consider, first, the force of the ^phmrical superficies FE which

OF NATURAL PHILOSOPHY.

22?

Sec. XII.]

is generated by the revolution of the arc PE,
and is cut any where, as in r, by the lineJe,
the annular part of the super 'icies generated
by the revolution of the arc rE will be as the
lineola D d, the radius of the sphere PE re- •>
maining the same; as Archimedes has de¬
monstrated in his Book of the Sphere and
Cylinder. And the force of this super¬
ficies exerted in the direction of the lines PE
or Pr situate all round in the conical superficies, will be as this annular
superficies itself; that is as the lineola D d, or, which is the same, as the
rectangle under the given radius PE of the sphere and the lineola Do? ; but
that force, exerted in the direction of the line PS tending to the centre S,
will be less in the ratio PD to PE, and therefore will be as PD X Do?.
Suppose now the line DP to be divided into innumerable little equal par¬
ticles, each of which call D d. and then the superficies PE will be divided
into so many equal annuli, whose forces will be as the sum of all the rec¬
tangles PD X D d, that is, as 1 PF 2 —^PD 2 , and therefore as DE 3 .
Let now the superficies FE be drawn into the altitude Ff; and the force
of the solid EF/e exerted upon the corpuscle P will be as DE 2 X Ff;
that is, if the force be given which any given particle as Ff exerts upon
the corpuscle P at the distance PF. But if that force be not given, the
force of the solid EF fe will be as the solid DE 2 X F/ and that force not
given, conjunctly. Q.E.D.

PROPOSITION LXXX. THEOREM XL.

If to the several equal parts of a sphere ABE described about the centre
S there tend equal centripetal forces ; and from the several points D
in the axis of the sphere AB in. which a corpuscle , as P, is placed ,
there be erected the perpendiculars DE meeting the sphere in E, and
if in those perpendiculars the lengths DN be taken as the quantity
DE 2 X PS

-pg-, and as tlu>force which a particle of the sphere situate in.

the axis exerts at the distance PE upon the corpuscle P conjunctly ; 1
say , that the whole force with which the corpuscle P is attracted to¬
wards the sphere is as the area ANB, comprehended under the axis of
the sphere AB, and the curve line ANB, the locus of the point N.

For supposing the construction in the last Lemma and Theorem to
stand, conceive the axis of the sphere AB to be divided into innumerable
equal particles Do?, and the whole sphere to be divided into so many sphe¬
rical concavo-convex laminae EF fe; and erect the perpendicular dn. By
the last Theorem, the force with which the laminae FtFfe attracts the cor¬
puscle P is as DE 2 X Ff and the force of one particle exerted at the

228

THE MATHEMATICAL PRINCIPLES

[Book I.

E<f

distance PE or PF, conjunctly.

But (by the last Lemma) D d is to

Fy* as PE to PS, and therefore Fy

PS X Do? , _
is equal to —^—; and DE 2 X

IB

PE

F/ is equal to D d X

DE 2 X PS
PE

and therefore the force of the la-
DE 2 X PS

mina EFyb is as Do? X

PE

and the force of a particle exerted at the distance PF conjunctly ; that is,
by the supposition, as DN X Do?, or as the evanescent area DNwtf.
Therefore the forces of all thelaminm exerted upon the corpuscle P are as
all the areas DN//c?, that is, the whole force of the sphere will be as the
whole area ANB. Q.E.D.

Cor. 1. Hence if the centripetal force tending to the several particles

DE 2 X PS

remain always the same at all distances, and DN be made as--;

PE

the whole force with which the corpuscle is attracted by the sphere is. as
the area ANB.

Cor. 2. If the centripetal force of the particles be reciprocally as the

DE 2 X PS

distance of the corpuscle attracted by it, and DN be made as--,

the force with which the corpuscle P is attracted by the whole sphere will
be as the area ANB.

Cor. 3. Jf the centripetal force of the particles be reciprocally as the
cube of the distance of the corpuscle attracted by it, and DN be made as
DE 2 X PS

—PEP- ; ^ orce the corpuscle is attracted by the whole

sphere will be as the area ANB.

Cor. 4. And universally if the centripetal force tending to the several
particles of the sphere be supposed to be reciprocally as the quantity V;
DE 2 X PS

and D5? be made as -; the force with which a corpuscle is at*

PE X V

tracted by the whole sphere will be as the area ANB.

PROPOSITION LXXXI. PROBLEM XLI.

The things remaining as above i it is required lo measure the area
ANB.

From the point P let there be drawn the right line PH touching the
sphere in H; and to the axis PAB, letting fall the perpendicular HI,
bisect PI in L; and (by Prop. XII, Book II, Elem.) PE 2 is equal t<r

Sec. XII.]

OF NATURAL PHILOSOPHY.

229

LD 2 — ALB. For LS 2 —

ALB xPS

-=-=--; where it instead oi V we write

PE X V

PS 2 + SE 3 + 2PSD. But because ^

the triangles SPH, SHI are alike,

SE 2 or SH 2 is equal to the rectan- \ \

gle PSI, Therefore PE 2 is equal ^/ \ \

to the rectangle contained under PS _1_X_

and PS + SI + 2SD ; that is, under L A k 1

PS and 2LS + 2SD ; that is, under V & J

PS and 2LD. Moreover DE 2 is S'

equal to SE 2 — SD 2 , or SE 2 — ^

LS 2 +2SLD— LD 2 , that is, 2SLD — LD 2 — ALB. For LS 2 —
SE 2 or LS 3 —SA a (by Prop. YI, Book II, Elem.) is equal to the rectan¬
gle ALB. Therefore if instead of EE 2 we write2SLD — LD 2 — ALB,
DE 2 X PS

the quantity —p^-—;—, which (by Cor. 4 of the foregoing Prop.) is as

the length of the ordinate DN, will now resolve itself into three parts

2SLD x PS LD 2 X PS ALB xPS , . r . x ,

— 77 ft— 77 —-ftft -i?- 7777 xT~ \ where it instead oi V we write

PE X Y PE X V PE X V

the inverse ratio of the centripetal force, and instead of PE the mean pro¬
portional between PS and 2LD, those three parts will become ordinates to
so many curve lines, whose areas are discovered by the common methods.
Q.E.D.

Example 1. If the centripetal force tending to the several particles of
the sphere be reciprocally as the distance; instead of Y write PE the dis
tance, then 2PS X LD for PE 2 ; and DN will become as SL — ( \ LD —

~|y Suppose DN equal to its double 2SL — LD ——j-g; and 2SL

the given part of the ordinate drawn into the length AB will describe the
rectangular area 2SL X AB; and the indefinite part LD, drawn perpen¬
dicularly into the same length with a continued motion, in such sort as in
its motion one way or another it may either by increasing or decreasing re-

LB 2 — LA 2

main always equal to the length LD, will describe the area-^-,

that is, the area SL X AB; which taken from the former area 2SL X

ALB

AB, leaves the area SL X AE. But the third part drawn after the

same manner with a continued motion perpendicularly into the same length,

will describe the area of an hyperbola, which subducted ^ ^

from the area SL X AB will leave ANB the area sought. yN

Whence arises this construction of the Problem. At

the points, L, A, B, erect the perpendiculars L /, A a, B b\

making A a equal to LB, and B6 equal to LA. Making

Li/ and LB asymptotes, describe through the points a, b , jJ—^-^

THE MATHEMATICAL PRINCIPLES

[Book 1

the hyperbolic crrve ab. And the chord ba being drawn, will inclose the
area aba equal to the area sought ANB.

Example 2. If the centripetal force tending to the several particles of
the sphere be reciprocally as the cube of the distance, or (which is the same

PE 3

thing) as that cube applied to any given plane; write ^gg for Y, and

SL X AS ^ AS ^

2PS X LD for PE 2 ; and DN will become as pg x pjj - £pg-

SxLB* * S (k ecause PS? AS, SI are continually proportional), as
— jSI — ^ we draw’ then these three parts into th

length AB, the first j-jj will generate the area of an hyperbola; the sec*

_ , . , ALB X SI , ALB X SL

ond iSI the area JAB X SI; the third — the area--

; that is, |AB X SI. From the first subduct the sum of the

2LB

second and third, and there will remain ANB, the area sought. Whence
arises this construction of the problem. At the points L, A, S, B, erect
l a the perpendiculars 12 A a Ss, B6, of which suppose Ss

l equal to SI; and through the point s, to the asymptotes

\ % LZ, LB, describe the hyperbola asb meeting the

\ s perpendiculars A a, B b, in a and b ; and the rectangle

-? 2ASI, subducted from the hyberbolic area A asbB, will

l. a~~i'— s- b leave ANB the area sought.

Example 3. If the centripetal force tending to the several particles of
the spheres decrease in a quadruplicate ratio of the distance from the par-

pp 4 _

tides; writer—— for V, then %/ 2PS + LD for PE, andDN will become
’ 2AS 3 ’

SI 2 X SL 1 SI 2 w 1 SI 2 X ALB v _ 1_

118 V2SI X v/LD 3 2^281 X v/LD 2v2SI ^LD 5 ‘
These three parts drawn into the length A B, produce so many areas, viz.

2SI 2 XSL . t — I “ 1~

V2SI mt ° v'LA v/ LB 5

SI 2 . -v-7- ,

1

1

~y~LA

7~LB 5

jB — v/

LA; and

1

0 v/LA 3

v/LB 3 '

V2SI v v 7

bSI 2 X ALB . j 1 1 ~~

3v/2SI ln ° y/LA 3 v/LB 3 ’
And these after due reduction come

forth L , SO, and SI» +

OF NATURAL PHILOSOPHY.

231

Sec. XII.]

2SI 3 . 4SI 3

-gj-j. And these by subducting the last from the first, become -^q.

Therefore the entire force with ,?hieh the corpuscle P is attracted towards

Si 3

the centre of the sphere is as-py, that is, reciprocally as PS 3 X PJ
Q.E.I.

By the same method one may determine the attraction of a corpuscle
situate within the sphere, but more expeditiously by the following Theorem.

PROPOSITION LXXXIL THEOREM XLI.

In a sphere described about the centre S with the interval SA, if there be
taken SI, SA, SP continually proportional; I say, that the attraction
of a corpuscle within the sphere in any place I is to its attraction without
the sphere in the place P in a ratio compounded of the subduplicate
ratio of IS, PS, the distances from the centre , and the subduplicate
ratio of the centripetal forces tending to the centre in those places P
and I.

As if the centripetal forces of the
particles of the sphere be reciprocally
as the distances of the corpuscle at¬
tracted by them ; the force with which
the corpuscle situate in I is attracted
by the entire sphere will be to the
force with which it is attracted in P
in a ratio compounded of the subdu¬
plicate ratio of the distance SI to the distance SP, and the subduplicate
ratio of the centripetal force in the place I arising from any particle in the
centre to the centripetal force in the place P arising from the same particle in
the centre; that is, in the subduplicate ratio of the distances SI, SP to each
other reciprocally. These two subduplicate ratios compose the ratio of
equality, and therefore the attractions in I and P produced by the whole
sphere are equal. By the like calculation, if the forces'of the particles of
the sphere are reciprocally in a duplicate ratio of the distances, it will be
found that the attraction in I is to the attraction in P as the distance SP
to the semi-diameter SA of the sphere. If those forces are reciprocally in
a triplicate ratio of the distances, the attractions in I and P will be to each
other as SP 2 to SA 2 ; if in a quadruplicate ratio, as SP 3 to SA 3 . There¬
fore since the attraction in P was found in this last case to be reciprocally
as PS 3 X PI, the attraction in I will be reciprocally as SA 3 X PI, that is,
because SA 3 is given reciprocally as PI. And the progression is the same
in infinitum. The demonstration of this Theorem is as follows:

The things remaining as above constructed, and a corpuscle being in anj

332

THE MATHEMATICAL PRINCIPLES

[Book I.

DE 2 X PS

place P. the ordinate DN was found to be as —prFr:— yf~. Therefore if

r ' rE X V

IE be drawn, that ordinate for any other place of the corpuscle, as I, will

x DE 2 X IS 1 .

become (mutatis mutandis) as y—. Suppose the centnpetaMbrces

flowing from any point of the sphere, as E, to be to each other at the dis¬
tances IE and PE as PE" to IE n (where the number u denotes the index

DE 2 X PS

of the powers of PE and IE), and those ordinates will become as
EE 2 X IS

and —--whose ratio to each other is as PS X IE X IE" to IS X

IE X IE n

PE X PE". Because SI, SE, SP are in continued proportion, the tri¬
angles SPE, SEI are alike; and thence IE is to PE as IS to SE or SA.
Fbr the ratio of IE to PE write the ratio of IS to SA; and the ratio of
the ordinates becomes that of PS X IE n to SA X PE". But the ratio of
PS to SA is subduplicate of that of the distances PS, SI; and the ratio of
IE" to PE" (because IE is to PE as IS to SA) is subduplicate of that of
the forces at the distances PS, IS. Therefore the ordinates, and conse¬
quently the areas which the ordinates describe, and the attractions propor¬
tional to them, are in a ratio compounded of those subduplicate ratios.
Q.E.D.

PROPOSITION LXXXIII. PROBLEM XLII.

To find the force with which a corpuscle placed in the centre of a sphere
is attracted towards any segment of that sphere whatsoever.

ri^ Let P be a body in the centre of that sphere and

RBSD a segment thereof contained under the plane
RDS, and thesphmrical superficies RBS. Let DB be cut
in F by a sphaerical superficies EFG described from the
centre P, and let the segment be divided into the parts
BREFGS, FEDG. Let us suppose that segment to
be not a purely mathematical but a physical superficies,
having some, but a perfectly inconsiderable thickness.
1 Let that thickness be called O, and (by what Archi -
medes has demonstrated) that superficies will be as
PF X T)F X O. Let us suppose besides the attrac¬
tive forces of the particles of the sphere to be reciprocally as that power of
she distances, of which n is index; and the force with which the superficies

DE 2 X O

EFG attracts the body P will be (by Prop. LXXIX) as ——, that

2DF X O DF 2 X O

is, as t

PF"

PF n

PF"

Let the perpendicular FN drawn into

Sec. XJ11.J of natural philosophy. 233

O be proportional to this quantity ; and the curvilinear area BDI, which
the ordinate FN, drawn through the length DB with a continued motion
will describe, will be as the whole force with which the whole segment
RBSD attracts the body P. Q.E.I.

PROPOSITION LXXXIY. PROBLEM XLIII.

To find the force with which a corpuscle, placed without the centre of a
sphere in the axis of any segment , is attracted by that segment.

Let the body P placed in the axis ADB of
the segment EBK be attracted by that seg¬
ment. About the centre P, witli the interval
PE, let the sphserical superficies EFK be de¬
scribed; and let it divide the segment into
two parts EBKFE and EFKDE. Find the
force of the first of those parts by Prop.

LXXXI, and the force of the latter part by
Prop. LXXXIII, and the sum of the forces will be the force of the whole
segment EBKDE. Q.E.I.

K

SCHOLIUM.

The attractions of sphaerical bodies being now explained, it comes next
in order to treat of the laws of attraction in other bodies consisting in like
manner of attractive particles; but to treat of them particularly is not neces¬
sary to my design. It will be sufficient to subjoin some general proposi¬
tions relating to the forces of such bodies, and the motions thence arising,
because the knowledge of these will be of some little use in philosophical
inquiries.

SECTION XIII.

Of the attractive forces of bodies which are not of a sphcerical figure,

PROPOSITION LXXXV. THEOREM XLII.

If a body be attracted by another , and its attraction be vastly stronger
when it is contiguous to the attracting body than when they are sepa¬
rated from one another by a very small interval; the forces of the
particles of the attract ing body decrease , in the recess cf the body at¬
tracted , in more than a duplicate ratio of the distance of the particles.
For if the forces decrease in a duplicate ratio of the distances from the
particles, the attraction towards a sphaerical body being (by Prop. LXXIV)
reciprocally as the square of the distance of the attracted body from the
sentre of the sphere, will not be sensibly increased by the contact, and it

234

THE MATHEMATICAL PRINCIPLES

[Book 1

will be still less increased by it, if the attraction, in the recess c*f the body
attracted, decreases in a still less proportion. The proposition, therefore,
is evident concerning attractive spheres. And the case is the same of con¬
cave sphmrical orbs attracting external bodies. And much more does it
appear in orbs that attract bodies placed within them, because there the
attractions diffused through the cavities of those, orbs are (by Prop. LXX)
destroyed by contrary attractions, and therefore have no effect even in the
place of contact. Now if from these spheres and sphaerical orbs we take
away any parts remote from the place of contact, and add new parts any
where at pleasure, we may change the figures of the attractive bodies at
pleasure; but the parts added or taken away, being remote from the place
of contact, will cause no remarkable excess of the attraction arising from
the contact of the two bodies. r J herefore the proposition holds good in
bodies of all figures. Q.E.D.

PROPOSITION LXXXVI. THEOREM XLIII.

If the forces of the particles of which cm attractive body is composed de¬
crease^ in the recess of the attractive body, in a triplicate or more than
a triplicate ratio of the distance from the particles, the attraction will
be vastly stronger in the point of contact than when the attracting and
attracted bodies are separated from each other, though by never so
small an interval.

For that the attraction is infinitely increased when the attracted corpus¬
cle comes to touch an attracting sphere of this kind, appears, by the solu¬
tion of Problem XLI, exhibited in fte second and third Examples. The
same will also appear (by comparing those Examples and Theorem XLI
together) of attractions of bodies made towards concavo-convex orbs, whether
the attracted bodies be placed without the orbs, or in the cavities within
them. And by aiding to or taking from those spheres and orbs any at¬
tractive matter any where without the place of contact, so that the attrac¬
tive bodies may receive any assigned figure, the Proposition will hold good
of all bodies universally. Q.E.D.

PROPOSITION LXXXVII. THEOREM XLIV.

If two bodies similar to each other, and consisting of matter equally at -
tractive i attract separately two corpuscles proportioned to those bodies ,
and in a like situation to them, the accelerative attractions of the cor¬
puscles toivards the entire bodies will be as the accelerative attractions
of the corpuscles towards particles of the bodies proportional to the
wholes, and alike situated in them.

For if the bodies are divided into particles proportional to the wholes,
and alike situated in them, it will be, as the attraction towards any parti¬
cle of one of the bodies to the attraction towards the correspondent particle

Sec. A III.]

OF NATURAL PHILOSOPHY.

235

in the other body, so are the attractions towards the several particles of the
first body, to the attractions towards the several correspondent particles of
the other body j and, by composition, so is the attraction towards the first
whole body to the attraction towards the second whole body. Q,.E.D.

Cor. 1 . Therefore if, as the distances of the corpuscles attracted increase,
the attractive forces of the particles decrease in the ratio of any power
of the distances, the accelerative attractions towards the whole bodies will
be as the bodies directly, and those powers of the distances inversely. As
if the forces of the particles decrease in a duplicate ratio of the distances
from the corpuscles attracted, and the bodies are as A 3 and B 3 , and there¬
fore both the cubic sides of the bodies, and the distance of the attracted
corpuscles from the bodies, are as A and B ; the accelerative attractions

A 3 B 3

towards the bodies will be as — and . that is, as A and B the cubic

A 2 B 2

sides of those bodies. If the forces of the particles decrease in a triplicate
ratio of the distances from the attracted corpuscles, the accelerative attrac-

A 3 B 3

tions towards the whole bodies will be as — and g~, that is, equal. If the

forces decrease in a quadruplicate ratio, the attractions towards the bodies

A 3 B 3

will be as — and —, that is, reciprocally as the cubic sides A and B.
And so in other cases.

Cor. 2. Hence, on the other hand, from the forces with which like bodies
attract corpuscles similarly situated, may be collected the ratio of the de¬
crease of the attractive forces of the particles as the attracted corpuscle
recedes from them; if so be that decrease is directly or inversely in any
ratio of the distances.

PROPOSITION LXXXVIII. THEOREM XLV.

If the attractive forces of the equal particles of any body be as the dis¬
tance of the places from the particles, the force of the whole body will
tend to its centre of gravity ; and will be the same with the force of
a globe, consisting of similar and equal matter , and having its centre
■in the centre of gravity.

Let the particles A, B, of the body RSTV at¬
tract any corpuscle Z with forces which, suppos¬
ing the particles to be equal between themselves,
are as the distances AZ, BZ; but, if they are
supposed unequal, are as those particles and
their distances AZ, BZ, conjunctly, or (if I may
so speak) as those particles drawn into their dis-

T

tancos AZ, BZ respectively. And let those forces be expressed by the

236 THE MATHEMATICAL PRINCIPLES [BOOK 1.

contents u.ider A X AZ, and B X BZ. Join AB, and let it be cut in G,
so that AG may be to BG as the particle B to the particle A ; and G
will be the common centre of gravity of the particles A and B. The force
A X AZ will (by Cor. 2, of the Laws) be resolved into the forces A X GZ
and A X AG; and the force B X BZ into the forces B X GZ and B X
BG. Now the forces A X AG and B X BG, because A is proportional to
B, and BG to AG, are equal, and therefore having contrary directions de¬
stroy one another. There remain then the forces A X GZ and B X GZ.

These tend from Z towards the centre G, and compose the force A + B
X GZ; that is, the same force as if the attractive particles A and B were
placed in their common centre of gravity G, composing there a little globe.

By the same reasoning, if there be added a third particle G, and the
force of it be compounded with the force A + B X GZ tending to the cen¬
tre G, the force thence arising will tend to the common centre of gravity
of that globe in G and of the particle C ; that is, to the common centre oi
gravity of the three particles A, B, C ; and will be the same as if that
globe and the particle C were placed in that common centre composing a
greater globe there; and so we may go on in infinitum. Therefore
the whole force of all the particles of any body whatever RSTV is the
same as if that body, without removing its centre of gravity, were to put
on the form of a globe. Q.E.D.

Cor. Hence the motion of the attracted body Z will be the same as if
the attracting body RSTV were sphaerical; and therefore if that attract¬
ing body be either at rest, or proceed uniformly in a right line, the body
attracted will move in an ellipsis having its centre in the centre of gravity
of the attracting body.

PROPOSITION LXXXIX. THEOREM XLVI.

If there be several bodies consisting of equal particles whose Jorces are
as the distances of the places from each, the force compounded of all
the forces by which any corpuscle is attracted will tend to the common
centre of gravity of the attracting bodies ; and will be the same as if
those attracting bodies, preserving their common centre of gravity,
should unite there, and be formed into a globe.

This is demonstrated after the same manner as the foregoing Proposi¬
tion.

Cor. Therefore the motion of the attracted body will be the same as if
the attracting bodies, preserving their common centre of gravity, should
unite there, and be formed into a globe. And, therefore, if the common
centre of gravity of the attracting bodies be either at rest, or proceed uni¬
formly in a* right line, the attracted body will move in an ellipsis having
its centre in the common centre of gravity of the attracting bodies.

Sec. Xill.j

OF NATURAL PHILOSOPHY.

237

D

PROPOSITION XC. PROBLEM XLIV.

If to the several points of any circle there tend equal centripeta forces ,
increasing or decreasing in any ratio of the distances ; it is required
to find the force icith which a corpuscle is attracted, that is, situate
any where in a right line which stands at right angles to the plane
of the circle at its centre.

Suppose a circle to be described about the cen¬
tre A with any interval AD in a plahe to which
the right line AP is perpendicular ; and let it be
required to find the force with which a corpuscle
P is attracted towards the same. From any point
E of the circle, to the attracted corpuscle P, let
there be drawn the right line PE. In the right
line PA take PF equal to PE, and make a per¬
pendicular FK, erected at F, to be as the force
with which the point E attracts the corpuscle P.

And let the curve line IKL be the locus of the point K. Let that cm re
meet the plane of the circle in L. In PA. take PH equal to PD, and er'/ct
the perpendicular HI meeting that curve in I; and the attraction of the
corpuscle P towards the circle will be as the area AHIL drawn into the
altitude AP. Q,.E.I.

For let there be taken in AE a very small line Ee. Join Pe, and in PE,
PA take PC, Pf equal to Pe. And because the force, with which any
point E of the annulus described about the centre A with the interval Afi
in the aforesaid plane attracts to itself the body P, is supposed to be as
FK; and, therefore, the force with which that point attracts the body P
AP X FK

towards A is as --; and the force with which the whole annulus

PE

attracts tne body P towards A is as the annulus and

AP X FK
PE“

conyunct-

ly; and that annulus also is as the rectangle under the radius A E and the
breadth Ee, and this rectangle (because PE and AE, Ee and CE are pro¬
portional) is equal to the rectangle PE X CE or PE X F f; the force
with which that annulus attracts the body P towards A will be as PE X
AP X FK

F/* and--conjunctly; that is, as the content under ¥f X FK X

srhi

AP, or as the area FK kf drawn into AP. And therefore the sum of the
forces with which all the annuli, in the circle described about the centre A
with the interval AD, attract the body P towards A, is as the whole area
AIHKL drawn into AP. Q..E.D.

Cor. 1. Hence if the forces of the points decrease in the duplicate ratio

238

THE MATHEMATICAL PRINCIPLES

[Book 1.

of the distances, that is, if FK be as jjpj, and therefore the area AHIKL
as p-r — p ; the attraction of the corpuscle P towards the circle will

, - PA AH

be as 1 — p|j; that is, as pg

Cor. 2. And universally if the forces of the points at the distances D bt
reciprocally as any power D n of the distances; that is, if FK be as

and therefore the area AHIKL as =r-r- 7 —

PA n — 1

of the corpuscle P towards the circle will be as

1

PH n — 15
1

PA' 1 —

the attraction
PA

PH"— r

Cor. 3. And if the diameter of the circle be increased in infinitum , and
the number n be greater than unity ; the attraction of the corpuscle P to¬
wards the whole infinite plane will be reciprocally as PA r ‘ — 2 , because the

other term

PA

PH n

vanishes.

PROPOSITION XCI. PROBLEM XLV.

To find the attraction of a corpuscle situate in the axis of a round solid ,
to whose several points there tend equal centripetal forces decreasing
in any ratio of the dista?ices whatsoever.

Let the corpuscle P, situate in the axis AB
of the solid DECG, be attracted towards that
solid. Let the solid be cut by any circle as
RFS, perpendicular to the axis j and in its
semi-diameter FS, in any plane PALKB pass¬
ing through the axis, let there be taken (by
Prop. XC) the length FK proportional to the
force with which the corpuscle P is attracted
towards that circle. Let the locus of the point
K be the curve line LKI, meeting the planes of the outermost circles AL
and BI in L and I; and the attraction of the corpuscle P towards the
solid will be as the area LABI. Q.E.I.

Cor. 1. Hence if the solid be a cylinder described by the parallelogram
A DEB revolved about the axis AB, and the centripetal forces tending to
the several points be reciprocally as the squares of the distances from the
points; the attraction of the corpuscle P towards this cylinder will be as
AB — PE + PD. For the ordinate FK (by Cor. 1, Prop. XC) will be
PF

as 1 — —— The part 1 of this quantity, drawn into the length AB, de-

Sec. X III-]

OF NATURAL PHILOSOPHY

239

3cribes the area 1 X AB ;
PF

pg-, drawn into the length

and the other part
PB describes the

E

area 1 into
shewn from

PE — AD (as may be
the quadrature of the

easily

curve

LKI); and, in like

manner,

the

drawn into the length PA describes

L into PD — AD,

same part
the area
and drawn into AB, the

-"IK-

onr

s

difference of PB and PA, describes 1 into PE —PD, the difference of the
areas. From the first content 1 X AB take away the last content 1 into
PE — PD, and there will remain the area LABI equal to 1 into
AB — PE 4* PD. Therefore the force, being proportional to this area,
is as AB — PE + PD.

Cor. 2. Hence also is known the force
by which a spheroid AGBC attracts any
body P situate externally in its axis AB. f
Let NKRM be a conic section whose or- fl
dinate HR perpendicular to PE may be
always equal to the length of the line PD
continually drawn to the point D in
which that ordinate cuts the spheroid.

From the vertices A, B, of the spheriod,
let there be erected to its axis AB the perpendiculars AK, BM, respectively
equal to AP, BP, and therefore meeting the conic sectio'n in K and M; and
join KM cutting off from it the segment KMRK. Let S be the centre of the
spheroid, and SC its greatest semi-diameter ; and the force with which the
spheroid attracts the body P will be to the force with which a sphere describ-

, . ATJ „ wl , , ASXCS--PSXKMRK

ed with the diameter AB attracts the same body as
AS 3

is to

3PS

PS 2 + CS 2 —AS 2
And by a calculation founded on the same principles may be

found the forces of the segments of the spheroid.

Cor. 3. If the corpuscle be placed within the spheroid and in its axis,
the attraction will be as its distance from the centre. This may be easily
collected from the following reasoning, whether
the particle be in the axis or in any other given
diameter. Let AGOF be an attracting sphe¬
roid, S its centre, and P the body attracted.

Through the body P let there be drawn the ! i
semi-diameter SPA, and two right lines DE, ^

FG meeting the spheroid in 1) and E, F and
G ; and let, PCM, HLN be the superficies of

240 the mathematical principle* £Book 1.

two interior spheroids similar and concentrical to the exterior, the first of
which passes through the body P. and cuts the right lines DE, FG in B
and C ; and the latter cuts the same right lines in H and I, K and L.
1 iet the spheroids have all one common axis, and the parts of the right
lines intercepted on both sides DP and BE, FP and CG, DH and IE, FK
and LG, will be mutually equal; because the right lines DE, PB, and HI,
are bisected in the same point, as are also the right lines FG, PC, and KL.
Conceive now DPF, EPG to represent opposite cones described with the
infinitely small vertical angles DPF, EPG, and the lines DH, El to be
infinitely small also. Then the particles of the cones DHKF, GLIE, cut
off by the spheroidical superficies, by reason of the equality of the lines DH
and El, will be to one another as the squares of the distances from the body
P, and will therefore attract that corpuscle equally. And by a like rea¬
soning if the spaces DPF, EGCB be divided into particles by the superfi¬
cies of innumerable similar spheroids concentric to the former and having
one common axis, all these particles will equally attract on both sides the
body P towards contrary parts. Therefore the forces of the cone DPF,
and of the conic segment EGCB, are equal, and by their contrariety de¬
stroy each other. And the case is the same of the forces of all the matter
that lies without the interior spheroid PCBM. Therefore the body P is
attracted by the interior spheroid PCBM alone, and therefore (by Cor. 3,
Prop. 1 .XXII) its attraction is to the force with which the body A is at¬
tracted by the whole spheroid AGOD as the distance PS to the distance
AS. Q.E.D.

PROPOSITION XCII. PROBLEM XLYI.

An attracting body being given , it is required to find the ratio of the de¬
crease of the centripetal forces tending to its several points.

The body given must be formed into a sphere, a cylinder, or some regu¬
lar figure, whose law of attraction answering to any ratio of decrease may
be found by Prop. LXXX, LXXXI, and XCI. Then, by experiments,
the force of the attractions must be found at several distances, and the law
of attraction towards the whole, made known by that means, will give
the ratio of the decrease of the forces of the several parts; which was to
be found.

PROPOSITION XCIII. THEOREM XLYII.

If a solid be plane on one side , and infinitely extended on all otljer sides ,
and consist of equal particles equally attractive , whose forces decrease ,
in the recess from the solid , in the ratio of any power greater than the
square of the distances ; and a corpuscle placed towards eithi r pa,rt of
the plane is attracted by the force of the whole solid ; I say that the
attractive force of the whole solid , in the recess from its plow superfi -

Sec. XIII.J

OF NATURAL PHILOSOPHY'.

241

ties, will decrease in the ratio of a power whose tide is the distance oj
the corpuscle from the plane, and its index less by 3 than the index of
the power of the distances.

CaseI. Let LG/be the plane by which
the solid is terminated. Let the solid .L

lie on that hand of the plane that is to-
wards I, and let it be resolved into in-_

numerable planes mHM, ?/IN, oKO,-

<fcc., parallel to GL. And first let the K l H 6

attracted body C be placed without the

solid. Let there be drawn CGHI per- o n m l

pendicular to those innumerable planes,

and let the attractive forces of the points of the solid decrease in the ratio
of a power of the distances whose index is the number n not less than 3.
Therefore (by Cor. 3, Prop. XC) the force with which any plane mHM
attracts the point C is reciprocally as CH n — 2 . In the plane mHM take the
length HM reciprocally proportional to CH’— 2 , and that force will be as
HM. In like manner in the several planes /GL, //TN, oKO, &c., take the
lengths GL, IN, KO, &c., reciprocally proportional to CG n — 2 , Cl 1 — 2 ,
CK n — 2 , &c., and the forces of tliose planes will be as the lengths so taken,
and therefore the sum of the forces as the sum of the lengths, that is, the
force of the w r hole solid as the area GLOK produced infinitely towards
OK. But that area (by the known methods of quadratures) is reciprocally
as CG n — 3 , and therefore the force of the whole solid is reciprocally as
CG n — 3 . Q.E.D.

Case 2. Let the corpuscle C be now placed on that
hand of the plane /GL that is within the solid, !o N L

and take the distance CK equal to the distance
CG. And the part of the solid LG/oKO termi¬
nated by the parallel planes /GL, oKO, will at- K I"”c G

tract the corpuscle C, situate in the middle, neither

one way nor another, the contrary actions of the ^

opposite points destroying one another by reason of

their equality. Therefore the corpuscle C is attracted by the force only
of the solid situate beyond the plane OK. But this force (by Case 1) is
reciprocally as CK n — 3 , that is, (because CG, CK are equal) reciprocally as
CG' 1 - 3 . Q.E.D.

Cor. 1. Hence if the solid LGIN be terminated on each side by two in¬
finite parallel places LG, IN, its attractive force is known, subducting
from the attractive force of the whole infinite solid LGKO the attractive
force of the more distant part NIKO infinitely produced towards KO.

Cor. 2. If the more distant part of this solid be rejected, because its at¬
traction compared wfith the attraction of the nearer part is inconsiderable^

16

242 THE MATHEMATICAL PRINCIPLES [BOOK 1

the attraction of that nearer part will, as the distance increases, decrease
nearly in the ratio of the power CG n — 3 .

Cor. 3. And hence if any finite body, plane on one side, attract a cor¬
puscle situate over against the middle of that plane, and the distance between
the corpuscle and the plane compared with the dimensions of the attracting
body be extremely small; and the attracting body consist of homogeneous
particles, whose attractive forces decrease in the ratio of any power of the
distances greater than the quadruplicate; the attractive force of the whole
body will decrease very nearly in the ratio of a power whose side is that
very small distance, and the index less by 3 than the index of the former
power. This assertion does not hold good, however, of a body consisting
of particles whose attractive forces decrease in the ratio of the triplicate
power of the distances; because, in that case, the attraction of the remoter
part of the infinite body in the second Corollary is always infinitely greater
than the attraction of the nearer part.

SCHOLIUM.

If a body is attracted perpendicularly towards a given plane, and from
the law of attraction given, the motion of the body be required ; the Pro¬
blem will be solved by seeking (by Prop. XXXIX) the motion of the body
descending in a right line towards that plane, and (by Cor. 2, of the Laws)
compounding that motion with an uniform motion performed in the direc¬
tion of lines parallel to that plane. And, on the contrary, if there be re¬
quired the law of the attraction tending towards the plane in perpendicu¬
lar directions, by which the body may be caused to move in any given
curve line, the Problem will be solved by working after the manner of the
third Problem.

But the operations may be contracted by resolving the ordinates into
converging series. As if to a base A the length B be ordinately ap¬
plied in any given angle, and that length be as any power of the base

A~; and there be sought the force with which a body, either attracted to¬
wards the base or driven from it in the direction of that ordinate, may be
caused to move in the curve line which that ordinate always describes with
its superior extremity; I suppose the base to be increased by a very small

part O, and I resolve the ordinate A + oi n into an infinite series A^ -f

— OA ----- 00 A &c., and I suppose the force propor-

n ahv

tional to the term of this series in which O is of two dimensions, that is,
to the term —5 -OOA —— Therefore the force sought is as

Sec. XIV.J

OF NATURAL PHILOSOPHY.

2A'.\

»

mm — mn m - 2n . . . . . . . mm — mn m - 2n

-A ~z —, or, which is the same thing, as-B .

As if the ordinate describe a parabola, m being = 2, and n = 1, the force
will be as the given quantity 2B°, and therefore is given. Therefore with
a given force the body will move in a parabola, as Galileo has demon¬
strated. If the ordinate describe an hyperbola, m being = 0 — 1, and n
— 1, the force will be as 2 A 3 or 2B 3 ; and therefore a force which is as the
cube of the ordinate will cause the body to move in an hyperbola. But
leaving this kind of propositions, I shall go on to some others relating to
motion which I have hot yet touched upon.

SECTION XIY.

Of the motion of very small bodies when agitated by centripetal forces
tending to the several parts of any very great body .

PROPOSITION XCIY. THEOREM XLYIII.

If two similar mediums be separated from each other by a space termi¬
nated on both sides by parallel planes , and a body in its passage
through that space be attracted or impelled perpendicularly towards
either of those mediums , and not agitated or hindered by any other
force ; and the attraction be every where the same at equal distances
from either plane, taken towai’ds the same hand of the plane; I say,
that the sine of incidence upon either plane will be to the sine of emcr
gence from the other plane in a given ratio.

Case 1. Let A a and B b be two parallel planes,
and let the body light upon the first plane A a in
the direction of the line GH, and in its whole
passage through the intermediate space let it be
attracted or impelled towards the medium of in¬
cidence, and by that action let it be made to de¬
scribe a curve line HI, and let it emerge in the di¬
rection of the line IK. Let there be erected IM
perpendicular to B£ the plane of emergence, and m

meeting the line of incidence GH prolonged in M, and the plane of inci¬
dence Aa in R; and let the line of emergence KI be produced and meet
HM in L. About the centre L, with the interval LI, let a circle be de¬
scribed cutting both HM in P and Q, and MI produced in N ; and, first,
if the attraction or impulse be supposed uniform, the curve IIT (by what
Galileo has demonstrated) be a parabola, whose property is that of a rec-

£44

THE MATHEMATICAL PRINCIPLES

[Book 1

MN, IR will be equal also,
n, and the rectangle NMI is

tangle under its given latus rectum and the line IM is equal to the squartf
of HM ; and moreover the line HM will be bisected in L. Whence if to
MI there be let fall the perpendicular LO, MO, OR will be equal; and
adding the equal lines ON, 01, the wholes
Therefore since IR is given, MN is also giv<
to the rectangle under the latus rectum and IM, that is, to HM 2 in a given
ratio. But the rectangle NMI is equal to the rectangle PMQ, that is, to
the ditference of the squares ML 2 , and PL 2 or LI 2 ; and HM 2 hath a given
ratio to its fourth part ML 2 ; therefore the ratio of ML 2 —LI 2 to ML 2 is given,
and by conversion the ratio of LI 2 to ML% and its subduplicate, theratrio
of LI to ML. But in every triangle, as LMI, the sines jf the angles are
proportional to the opposite sides. Therefore the ratio of the sine of the
angle of incidence LMR to the sine of the angle of emergence LIR is
given. Q,.E.P.

Case 2. Let now the body pas3 successively through several spaces ter¬
minated with parallel planes Aa/>B, B6cC, &c., and let it be acted on by a
\ . force which is uniform in each of them separ-

A \ _ a ately, but different in the different spaces; and

_ b by what was just demonstrated, the sine of the

.X

£ angle of incidence on the first plane A a is to
^ the sine of emergence from the second plane B6

in a given ratio; and this sine of incidence upon the second plane B b will
be to the sine of emergence from the third plane C c in a given ratio; and
this sine to the sine of emergence from the fourth plane T>d in a given ra¬
tio ; and so on in infinitum ; and, by equality, the sine of incidence on
the first plane to the sine of emergence from the last plane in a given ratio.
I iet now the intervals of the planes be diminished, and their number be in¬
finitely increased, so that the action of attraction or impulse, exerted accord¬
ing to any assigned law, may become continual, and the ratio of the sine of
incidence on the first plane to the sine of emergence from the last plane
being all along given, will be given then also. Q,.E.D.

PROPOSITION XCY. THEOREM XLIX.

The same things being supposed , I say , that the velocity of the body be¬
fore its incidence is to its velocity after emergence as the sine of emer¬
gence to the sine of incidence.

Make AH and I d equal, and erect the perpen¬
diculars AG, dK meeting the lines of incidence
and emergence GH, IK, in G and K. In GH
take TH equal to IK, and to the plane A a let
fall a perpendicular Tv. And (by Cor. 2 of the
Laws of Motion) let the motion of the body be
resolved into two, one perpendicular to the planes

OF NATURAL PHILOSOPHY.

245

Sec. XIV.]

A a, B6, C c, foe, and another parallel to them. The force of attraction or
impulse ; acting in directions perpendicular to those planes, does not at all
alter the motion in parallel directions; and therefore the body proceeding
with this motion will in equal times go through those equal parallel inter¬
vals that lie between the line AG and the point H, and between the point
I and the line dK ; that is, they will describe the lines GH, IK in equal
times. Therefore the velocity before incidence is to the velocity after
emergence as GH to IK or TH, that is, as AH or Id to vH, that is (sup¬
posing TH or IK radius), as the sine of emergence to the sine of inci¬
dence. Q.E.D.

aT

-a

-6

~ c d

-c

PROPOSITION XOV1. THEOREM L.

7 Vie same things being supposed , and that the motion before incidence is
swifter than aftenoards ; 1 say, that if the line of incidence be in¬
clined continually , the body will be at last reflected , and the angle of
reflexion will be equal to the angle of incidence.

For conceive the body passing between the parallel planes A a, B6, Cc,
foe., to describe parabolic arcs as above; sg
and let those arcs be HP, PQ,, Q,R, foe.

And let the obliquity of the line of inci- g -
dence GH to the first plane A a be such R
that the sine of incidence may be to the radius of the circle whose sine it is,
in the same ratio which the same sine of incidence hath to the sine of emer¬
gence from the plane D d into the space DcfeE ; and because the sine of
emergence is now become equal to radius, the angle of emergence will be a
right one, and therefore the line of emergence will coincide with the plane
Dd. Let the body come to this plane in the point R; and because the
line of emergence coincides with that plane, it is manifest that the body can
proceed no farther towards the plane Ee. But neither can it proceed in the
line of emergence Rc£; because it is perpetually attracted or impelled towards
the medium of incidence. It will return, therefore, between the planes Cc,
Dd, describing an arc of a parabola QR</, whose principal vertex (by what
Galileo has demonstrated) is in R_, cutting the plane Cc in the same angle
at q, that it did before at Q,; then going on in the parabolic arcs qp, ph,
&c., similar and equal to the former arcs QP, PH, &c., it will cut the rest
of the planes in the same angles at p, h , (fee., as it did before in P, H, (fee.,
and will emerge at last with the same obliquity at h with which it first
impinged on that plane at H. Conceive now the intervals of the planes
A a, B b, Cc, D d, Ee, foe., to be infinitely diminished, and the number in¬
finitely increased, so that the action of attraction or impulse, exerted ac¬
cording to any assigned law, may become continual; and, the angle of
emergence remaining all alor g equal to the angle of incidence, will be
equal to the same also at last. Q.E.D.

246

THE MATHEMATICAL PRINCIPLES

IBook I

SCHOLIUM.

These attractions bear a great resemblance to the reflexions and refrac¬
tions of light made in a given ratio of the secants, as was discovered hj
Snellius ; and consequently in a given ratio of the sines, as was exhibited
by Des Cartes. For it is now certain from the phenomena of Jupiter 3 s
isatellites, confirmed by the observations of different astronomers, that light
is propagated :n succession, and requires about seven or eight minutes to
travel from the sun to the earth. Moreover, the rays of light that are in
our air (as lately was discovered by Grimaldns , by the admission of light
into a dark room through a small hole, which 1 have also tried) in their
passage near the angles of bodies, whether transparent or opaque (such a3
the circular and rectangular edges of gold, silver and brass coins, or of
knives, or broken pieces of stone or glass), are bent or inflected round those
bodies as if they were attracted to them ; and those rays which in their
passage come nearest to the bodies are the most inflected, as if they were
most attracted ; which thing I myself have also carefully observed. And
those which pass at greater distances are less inflected; and those at still
greater distances are a little inflected the contrary way, and form three
fringes of colours. In the figure 5 represents the edge of a knife, or any

A B

/

yf 6

a

kind of wedge AsB ; and gowog, fnunf, emtme, dlsld , are rays inflected to¬
wards the knife in the arcs own, nvn, mtm, 1st ; which inflection is greater
or less according to their distance from the knife. Now since this inflec¬
tion of the rays is performed in the air without the knife, it follows that the
rays which fall upon the knife are first inflected in the air before they touch
the knife. And the case is the same of the rays falling upon glass. The
refraction, therefore, is made not in the point of incidence, but gradually, by
a continual inflection of the rays; which is done partly in the air before they
touch the glass, partly (if [ mistake not) within the glass, after they have
entered it; as is represented in the rays ckzc,, bit/b, ahxa , falling upon r,
q,p, and inflected between k and z, i and y, h and x. Therefore because
of the analogy there is between the propagation of the rays f light and the
motion of bodies, I thought it not amiss to add the followi ig Propositions
fur optical uses ; not at all considering the nature of the rays of light, or
inquiring whether they are bodies or not; but only determining the tra*
jectories of bodies which are extremely like the trajectories of the rays.

Sec. XIV.]

OF NATURAL PHILOSOPHY.

247

PROPOSITION XCVII. PROBLEM XLVI1.

Supposing the sine of incidence upon any superficies to be in a given ra¬
tio to the sine of emergence ; and that the inflection of the paths of
those bodies near that superficies is performed in a very short space ,
which may be considered as a point; it is required to determine such
a superficies as may cause all the corpuscles issuing from any one
given place to con verge to another given place.

Let A be the place from whence the cor¬
puscles diverge ; B the place to which they
should converge; CDE the curve line which
by its revolution round the axis AB describes A C nm

the superficies sought; D, E, any two points of that curve ; and EF, EG,
perpendiculars let fall on the paths of the bodies AD, DB. Let the point
D approach to and coalesce with the point E; and the ultimate ratio of
the line DF by which AD is increased, to the line DG by which DB is
diminished, will be the same as that of the sine of incidence to the sine of
emergence Therefore the ratio of the increment of the line AD to the
decrement of the line I)B is given; and therefore if in the axis AB there
be taken any where the point C through which the curve CDE must
pass, and CM the increment of AC be taken in that given ratio to CN
the decrement of BC, and from the centres A, B, with the intervals AM,
BN, there be described two circles cutting each other in D; that point D
will touch the curve sought CDE, and, by touching it any where at pleasure,
will determine that curve. Q,.E.I.

Cor. 1. By causing the point A or B to go off sometimes in infinitum,
and sometimes to move towards other parts of the point C, will be obtain¬
ed all those figures which Cartesins has exhibited in his Optics and Geom¬
etry relating to refractions. The invention of which Cartesins having
thought fit to conceal, is here laid open in this Proposition.

Cor. 2. If a body lighting on any superfi¬
cies CD in the direction of a right line AD,
drawn according to any law, should emerge
in the direction of another right line DK;
and from the point C there be drawn curve

lines CP, CQ,, always perpendicular to AD, DK ;
lines PD, QD, and therefore the lines themselves
those increments, will be as the sines of incidence and emergence to
other, and e contra.

the increments of the
PD, Q.D, generated by

PROPOSITION XCVIII. PROBLEM XLVIII.

The same things supposed ; if round the axis AB any attractive super¬
ficies be described as CD, regular or irregular, through which the bo¬
dies issuing from the given place A must pass ; it is required to find

24S

THE MATHEMATICAL PRINCIPLES.

[Book 1

a second attractive superficies EF, which may make those bodies con •
verge to a given place B.

Let a line joining AB cut
the lirst superficies in C and
the second in E, the point D
being taken any how at plea¬
sure. And supposing the
i G sine of incidence on the first
superficies to the sine of
emergence from the same, and the sine of emergence from the second super¬
ficies to the sine of incidence on the same, to be as any given quantity M
to another given quantity N; then produce AB to G, so that BG may he
to CE as M — N to N; and AD to H, so that AH may be equal to AG ;
and DF to K, so that DK may be to DH as N to M. Join KB, and about
the centre D with the interval DH describe a circle meeting KB produced
in L, and draw BF parallel to DL; and the point F will touch the line
EF, which, being turned round the axis AB, will describe the superficies
sought. Q,.H.F.

For conceive the lines CP, CQ to be every where perpendicular to AD,
DF, and the lines ER, ES to FB, FD respectively, and therefore Q.S to
be always equal to CE; and (by Cor. 2, Prop. X.CVII) PD will be to QD
as M to N, and therefore as DL to DK, or FB to FK ; and by division as
DL — FB or PH — PD — FB to FD or FQ — QD ; and by composition
as PH — FB to FQ, that is (because PH and CG, QS and CE, are equal),
as CE -f BG — FR to CE — FS. But (because BG is to CE as M —
N to N) it. comes to pass also that CE + BG is to CE as M to N; and
therefore, by division, FR is to FS as M to N; and therefore (by Cor. 2,
Prop XCVI1) the superficies EF compels a body, falling upon it in the
direction DF, to go on in the line FR to the place B. Q.E.D.

SCHOLIUM.

.In the same manner one may go on to three or more superficies. But
of all figures the sphaerical is the most proper for optical uses. If the ob¬
ject glasses of telescopes were made of two glasses of a sphaerical figure,
containing water between them, it is not unlikely that the errors of the
refractions made in the extreme parts of the superficies of the glasses may
be accurately enough corrected by the refractions of the water. Such ob¬
ject glasses are to be preferred before elliptic and hyperbolic glasses, not only
because they may be formed with more ease and accuracy, but because the
pencils of rays situate without the axis of the glass would be more accu¬
rately refracted by them. But the different refrangibility of different raya
is the real obstacle that hinders optics from being made perfect by sphaeri¬
cal or any other figures. Unless the errors thence arising can be corrected,
all the labour spent in correcting the others is quite thrown away.

BOOK II.

OF THE MOTION OF BODIES.

SECTION L

Of the motion of bodies that are resisted in the ratio of the velocity .

PROPOSITION I. THEOREM I.

Tf a body is resisted in the ratio of its velocity , the motion lost by re¬
sistance is as the space gone over in its motion.

For since the motion lost in each equal particle of time is as the velocity,
that is, as the particle of space gone over, then, by composition, the motion
lost in the whole time will be as the whole space gone over. Q.E.D.

Cor. Therefore if the body, destitute of all gravity, move by its innate
force only in free spaces, and there-be given both its whole motion at the
beginning, and also the motion remaining after some part of the way is
gone over, there will be given also the whole space which the body can de¬
scribe in an infinite time. For that space will be to the space now de¬
scribed as the whole motion at the beginning is to the part lost of that
motion.

LEMMA I.

Quantities proportional to their differences are continually proportional .

Let A be to A — B as B to B — C and C to C — D, (fee., and, by con¬
version, A will be to B as B to C and C to D, (fee. Q.E.D.

PROPOSITION II.- THEOREM II.

If a body is resisted in the ratio of its velocity , and moves , by its vis in-
sita only } through a similar medium , and the times be taken equal ,
the velocities in the beginning of each of the times are in a geometri¬
cal progression , and the spaces described in each of the times are as
the velocities .

Case 1. Let the time be divided into equal particles; and if at the very
beginning of each particle we suppose the resistance to act witli one single
impulse which is as the velocity, the decrement of the velocity in each of

252

THE MATHEMATICAL PRINCIPLES

[Book II.

the particles of time will be as the same velocity. Therefore the veloci¬
ties are proportional to their differences, and therefore (by Lem. 1, Book
II) continually proportional. Therefore if out of an equal number of par¬
ticles there be compounded any equal portions of time, the velocities at the
beginning of those times will be as terms in a continued progression, which
are taken by intervals, omitting every where an equal number of interme¬
diate terms. But the ratios of these terms are compounded of the equa J
ratios of the intermediate terms equally repeated, and therefore are equal
Therefore the velocities, being proportional to those terms, are in geomet¬
rical progression. Let those equal particles of time be diminished, and
their number increased in infinitum, so that the impulse of resistance may
become continual; and the velocities at the beginnings of equal times, al¬
ways continually proportional, will be also in this case continually pro¬
portional. Q.E.D.

Case 2. And, by division, the differences of the velocities, that is, the
parts of the velocities lost in each of the times, are as the wholes; but the
spaces described in each of the times are as the lost parts of the velocities
(by Prop. 1, Book I), and therefore are also as the wholes. Q.E.D.

Corol. Hence if to the rectangular asymptotes AC, CH,
the hyperbola BG is described, and AB, DG be drawn per-
-g pendicular to the asymptote AC, and both the velocity of
^ the body, and the resistance of the medium, at the very be¬
ginning of the motion, be expressed by any given line AC,
and, after some time is elapsed, by the indefinite line DC; the time may
be expressed by the area ABGD, and the space described in that time by
the line AD. For if that area, by the motion of the point D, be uniform¬
ly increased in the same manner as the time, the right line DC will de¬
crease in a geometrical ratio in the same manner as the velocity; and the
parts of the right line AC, described in equal times, will decrease in the
same ratio.

PROPOSITION III. PROBLEM I.

To define the motion o f a body which , in a similar medium , ascends or
descends in a right line , and is resisted in the ratio of its velocity , and

acted upon by an uniform force of gravity.

The body ascending, let the gravity be expound¬
ed by any given rectangle BACH; and the resist¬
ance of the medium, at the beginning of the ascent,
by the rectangle BADE, taken on the contrary side
of the right line AB. Through the point B, with
the rectangular asymptotes AC, CH, describe an
hyperbola, cutting the perpendiculars DE, de, in

OF NATURAL PHILOSOPHY.

253

Sec, I.J

G, g ; and the body ascending will in the time DGgd describe the space
E Gge; in the time DGBA, the space of the whole ascent EGB; in the
time ABK1, the space of descent BFK; and in the time IK ki the space of
descent KF/k; and the velocities of the bodies (proportional to the re¬
sistance of the medium) in these periods of time will be ABED, AB ed, O,
ABPI, AB/i respectively; and the greatest velocity which the body can
acquire by descending will be BACH.

For let the rectangle BACH be resolved into in¬
numerable rectangles A k, Kl, L m, M/q tfea, which
shall be as the increments of the velocities produced
in so many equal times; then will 0, A k, A l. Am, An,

&c., be as the whole velocities, and therefore (by suppo¬
sition) as the resistances of the medium in the be¬
ginning of each of the equal times. Make AC to
AK, or ABHC to AB/cK, as the force of gravity to the resistance in the
beginning of the second time; then from the force of gravity subduct the
resistances, and ABHC, K£HC, LZHC, M/AHC, &c., will be as the abso¬
lute forces with which the body is acted upon in the beginning of each of
the times, and therefore (by Law I) as the increments of the velocities, that
is, as the rectangles A k, Kl, L m, M//, (fee., and therefore (by Lem. 1, Book
II) in a geometrical progression. Therefore, if the right lines K k, L/
M m, N n, (fee., are produced so as to meet the hyperbola in q, r, s, t, (fee.,
the areas AB^K, Kqrh, LrsM, MsfN, (fee., will be equal, and there¬
fore analogous to the equal times and equal gravitating forces. But the
area AB<?K (by Corol. 3, Lem. VII and VIII, Book I) is to the area BA:^
as to \kq, or AC to |AK, that is, as the force of gravity to the resist¬
ance in the middle of the first time. And by the like reasoning, the areas
qKKr, rLMs, sMN£, (fee., are to the areas qklr, rims, smnt, (fee., as the
gravitating forces to the resistances in the middle of the second, third, fourth
time, and so on. Therefore since the equal areas BAKy, qKhr, rLMs,
sMNt, (fee., are analogous to the gravitating forces, the areas B kq, qklr,
rims, smut, (fee., will be analogous to the resistances in the middle of
each of the times, that is (by supposition), to the velocities, and so to the
spaces described. Take the sums of the analogous quantities, and the areas
B kq, B Ir, B ms, But, (fee., will be analogous to the whole spaces described;
and also the areas ABqK, ABrL, ABsM, ABtfN, (fee., to the times. There¬
fore the body, in descending, will in any time ABrL describe the space Blr,
and in the time LrtN the space rlnt. Q.E.D. And the like demonstra¬
tion holds in ascending motion.

Corol. 1. Therefore the greatest velocity that the body can acquire by
falling is to the velocity acquired in any given time as the given force ot
gravity which perpetually acts upon it to the resisting force which opposes
it at the end of that time.

854

THE MATHEMATICAL PRINCIPLES

[Book IL

Corol. 2. But the time being augmented in an arithmetical progression,
the sum of that greatest velocity and the velocity in the ascent, and also
their difference in the descent, decreases in a geometrical progression.

Corol. 3. Also the differences of the spaces, which are described in equal
differences of the times, decrease in the same geometrical progression.

Corol. 4. The space described by the body is the difference of two
spaces, whereof one is as the time taken from the beginning of the descent,
and the other as the velocity* which [spaces] also at the beginning of the
descent are equal among themselves.

PROPOSITION IV. PROBLEM II.

Supposing the force of gravity in any similar medium to be uniform,
and to tend perpendicularly to the plane of the horizon; to define the
motion of a projectile therein, which suffers resistance proportional to
its velocity.

2*f Let the projectile go from any place D in
/ the direction of any right line DP, and let
/ its velocity at the beginning of the motion
/ he expounded by the length DP. From the
/ point P let fall the perpendicular PC on the

/ horizontal line DC, and cut DC in A, so

/ that DA may be to AC as the resistance

/ of the medium arising from the motion up-

"Xr ~y B wards at the beginning to the force of grav-

37 / ity; or (which comes to the same) so that

/ / t ie rectangle under DA and DP may be to

-3L / / that under AC and CP as the whole resist-

/ ^-Ei ance at the beginning of the motion to the

/ / force of gravity. With the asymptotes

•g. / II DC, CP describe any hyperbola GTBS cut-

G \ L ting the perpendiculars DG, AB in G and

p _ \ \g B ; complete the parallelogram DGKC, and

D BA. let s id e GK cut AB in Q. Take a line

N in the same ratio to Q,B as DC is in to CP; and from any point R of the
right line DC erect RT perpendicular to it, meeting the by] erbola in T,
and the right lines EH, GK, DP in I, t , and Y; in that perpendicular

2GT v

take Yr equal to —or which is the same thing, take Rr equal to

GTIE

—; and the projectile in the time DRTG will arrive at the point r

describing the curve line DraF, the locus of the point r ; thence it will
come to its greatest height a in the perpendicular AB; and afterwards

Sec. 1.J

OF NATURAL PHILOSOPHY.

255

ever approach to the asymptote PC. And its velocity in any pjint r will
be as the tangent rL to the curve. Q.E.I.

For N is to QB as DC to CP or DR to RV, and therefore RV is equal to

DRXQB , „ DR x QB-*GT N .

-, and Rr (that is, RV — Vr, or-^-) is equal to

N

DR X AB — RDGT _ , , . , , , ,

-^-. Now let the time be expounded by the area

RDGT and (by Laws, Cor. 2), distinguish the motion of the body into
two others, one of ascent, the other lateral. And since the resistance is as
the motion, let that also be distinguished into two parts proportional and
contrary to the parts of the motion : and therefore the length described by
the lateral motion will be (by Prop. II, Book II) as the line DR, and the
height (by Prop. Ill, Book II) as the area DR X AB — RDGT, that is.
as the line Rr. But in the very beginning of the motion the area RDGT
is equal to the rectangle DR X AQ, and therefore that line Rr (or

jjj!: DR * ^~— ) w m then be to DR as AB — AQ or QB to N,

that is, as CP to DC ; and therefore as the motion upwards to the motion
lengthwise at the beginning. Since, therefore, Rr is always as the height,
and DR always as the length, and Rr is to DR at the beginning as the
height to the length, it follows, that Rr is always to DR as the height to
the length ; and therefore that the body will move in the line DraF, which
is the locus of the point r. Q.E.D.

^ ^ . DR X AB RDGT , , „

Cor. 1. Therefore Rr is equal to -^-^— . and therefore

N

N

if RT be produced to X so that RX may be equal to ——,, that is,

if the parallelogram ACPY be completed, and DY cutting CP in Z be
drawn, and RT be produced till it meets DY in X; Xr will be equal to

RDGT , , , . ,

—N— an( * ™ erc * ore proportional

Cor. 2. Whence if innumerable lines CR, or, which is the same, innu¬
merable lines ZX, be taken in a geometrical progression, there will be as
many lines Xr in an arithmetical progression. And hence the curve DraF
is easily delineated by the table of logarithms.

Cor. 3. If a parabola be constructed to the vertex D, and the diameter
DG produced downwards, and its latus rectum is to 2 DP as the whole
resistance at the beginning of the notion to the gravitating force, the ve¬
locity with which the body ought *o go from the place D, in the direction
of the right line DP, so as in an uniform resisting medium to describe the
curve DraF, will be the same as that with which it ought to go from the
same place D in the direction of the same right line DP, so as to describe

256

THE MATHEMATICAL PRINCIPLES *.

[Book II

a parabola in a non-resisting medium. For
the latus rectum of this parabola, at the very

DY 2

beginning of the motion, is y - ; and Yris
*GT DR XT t

or-—. But a right line, which,

if drawn, would touch the hyperbola GTS in

G, is parallel to DK, and therefore T£ is

CKxDR JAT . QBxDC 4
--, and JN is--. Ahd there¬

fore Yr is equal to

X CK X CP

2DC 2 X Q,B

, that is (because DR and DC, DY

and DP are proportionals), to

DY 2 X CK X CP

and the latus rectum

**■*■'' r r— — n ^ gDP x Q.B J xwwi**x*

DY 2 2DP 2 X Q,B , . '

-yy - COmeS <>Ut CK X ~CP~ ? 1S (° eCaUSe aD( ^ CK, an< ^ AC
.. 2DP 2 X DA , ,

are proportional), x qp ’ > ana therefore 1S to 2DP as DP X DA to

CP X AC; that is, as the resistance to the gravity. Q.E.D.

2!/ Cor. 4. Hence if a body be projected from
/ any place D with a given velocity, in the
/ direction of a right line DP given by posi-

/ tion, and the resistance of the medium, at

/ the beginning of the motion, be given, the

/ curve DraF, which that body will describe,

/ may be found. For the velocity being

/ given, the latus rectum of the parabola is

~Yh- -y P given, as is well known. And taking 2DP

Xl / to that latus rectum, as the force of gravity

/ / to the resisting force, DP is also given.

-5L / s Then cutting DC in A, so that CP X AC

/ -tj/ J ® L may be to DP X DA in the same ratio of
/ / the gravity to the resistance, the point A

j. / ^ will be given. And hence the curve DraF

q \ ^ is also given.

If ~£\Q \ k Cor. 5. And, on the contrary, if the

H RA. S' curve DraF be given, there will be given

loth the velocity of the body and the resistance of the medium in each of
the places r. For the ratio of CP X AC to DP X DA being given, there
is given both the resistance of the medium at the beginning of the motion,
and the latus rectum of the parabola; and thence the velocity at the be¬
ginning of the motion is given also. Then from the length of the tangent

OF NATURAL PHILOSOPHY.

257

Sec. I.]

L there is given both the velocity proportional to it, and the resistance
proportional to the velocity in any place r.

Cor. 6 . But since the length 2DP is to the latus rectum of the para¬
bola as the gravity to the resistance in D; and, from the velocity aug¬
mented, the resistance is 'u gmented in the same ratio, but the latus rectum
of the parabola is augmented in the duplicate of that ratio, it is plain thot
the length 2DP is augmented in that simple ratio only ; and i3 therefore
always proportional to the velocity ; nor will it be augmented or dimin¬
ished by the change of the angle CDP, unless the velocity be also changed.

Cor. 7. Hence appears the method of deter¬
mining the curve DmF nearly from the phe¬
nomena, and thence collecting the resistance and
velocity with which the body is projected. Let
two similar and equal bodies be projected with
the same velocity, from the place D, in differ¬
ent angles CDP, CD/?; and let the places F,
f. where they fall upon the horizontal plane
DC, be known. Then taking any length for ®

DP or D ] p suppose the resistance in D to be to
the gravity in any ratio whatsoever, and let that
ratio be expounded by any length SM. Then,
by computation, from that assumed length DP,
find the lengths DF, D f; and from the ratio
Ff

pp, found by calculation, subduct the same ratio as found by experiment;

MM

M

and let the (difference be expounded by the perpendicular MN. Repeat the
same a second and a third time, by assuming always a new ratio SM of the
resistance to the gravity, and collecting a new difference MN. Draw the
affirmative differences on one side of the right line SM, and the negative
on the. other side; and through the points N, N, N, draw a regular curve
NNN, cutting the right line SMMM in X, and SX will be the true ratio
of the resistance to the gravity, which was to be found. From this ratio
the length DF is to be collected by calculation; and a length, which is to
the assumed length DP as the length DF known by experiment to the
length DF just now found, will be the true length DP. This being known,
you will have both the curve line D/v*F which the body describes, and also
the velocity and resistance of the body in each place.

SCHOLIUM.

But, yet, that the resistance of bodies is in the ratio of the velocity, is more
a mathematical hypothesis than a physical one. In mediums void of all te¬
nacity, the resistances made to bodies are in the duplicate ratio of the ve¬
locities. For by the action of a swifter body, a greater motion in propor-

17

253

THE MATHEMATICAL PRINCIPLES

[Book IL

tion to a greater velocity is communicated to the same quantity of the
medium in a less time; and in an equal time, by reason of a greater quan¬
tity of the disturbed medium, a motion is communicated in the duplicate
ratio greater; and the resistance (by Law II and III) is as the motion
communicated. Let us, therefore, see what motions arise from this law of
resistance.

SECTION II.

)f the motion of bodies that are resisted in the duplicate ratio of their

velocities.

PROPOSITION V. THEOREM III.

If ci body is resisted in the duplicate ratio of its velocity , and moves by
its innate force only through a similar medium; and the times be
taken in a geometrical progression ., proceeding from less to greater
terms : I say , that the velocities at the beginning of each of the times
are in the same geometrical progression inversely ; and that the spaces
are equal , which are described in each of the times.

For since the resistance of the medium is proportional to the square of
the velocity, and the decrement of the velocity is proportional to the resist¬
ance : if the time be divided into innumerable equal particles, the squares of
the velocities at the beginning of each of the times will be proportional to
the differences of the same velocities. Let those particles of time be AK,
KL, LM, &c., taken in the right line CD; and
erect the perpendiculars AB, KA;, L l, Mm, &c.,
meeting the hyperbola BA;/mG, described with the
centre C, and the rectangular asymptotes CD, CH,
in B, k, l, m, &c.; then AB will be to KA; as CK
to CA, and, by division, AB —KA; to KA; as AK
to CA, and alternately, AB — Kk to AK as Kk
to CA; and therefore as AB X K k to AB X CA.
Therefore since AK and AB X CA are given,'AB — KA; will be as AB
X KA;; and, lastly, when AB and KA; coincide, as AB 2 . And, by the like
reasoning, K k—hl, U—Mm, (fee., will be as Kk 2 . LI 2 , (fee. Therefore the
squares of the lines AB, KA*, LI, M m, (fee., are as their differences; and,
therefore, since the squares of the velocities were shewn above to be as their
differences, the progression of both will be alike. This being demonstrated
it follows also that the areas described by these lines are in a like progres¬
sion with the spaces described by these velocities. Therefore if the velo¬
city at the beginning of the first time AK bo expounded by the line AB,

OF NATURAL PHILOSOPHY.

Sec. II.]

oxu

and the velocity at the beginning of the second time KL by the line KA
and the length described in the first time by the area AKArB, all the fol¬
lowing velocities will be expounded by the following lines U, Mm, &c.
and the lengths described, by the areas K/, I mi. &e. And, by compo¬
sition, if the whole time be expounded by AM, the sum of its parts, the
whole length described will be expounded by AMmB the sum of its parts.
Now conceive the time AM to be divided into the parts AK, KL, LM, (fee
so that CA, CK. CL, CM, (fee. may be in a geometrical progression; and
those parts will be in the same progression, and the velocities AB, K/r,
L l, M m, (fee., will be in the same progression inversely, and the spaces de¬
scribed A k, K/, L m, (fee., will be equal. Q..E.D.

Cor. 1. Hence it appears, that if the time be expounded by any part
AD of the asymptote, and the velocity in the beginning of the time by the
ordinate AB, the velocity at the end of the time will be expounded by the
ordinate DG; and the whole space described by the adjacent hyperbolic
area ABGD ; and the space which any body can describe in the same time
AD, with the first velocity AB, in a non-resisting medium, by the rectan¬
gle AB X AD.

Cor 2. Hence the space described in a resisting medium is given, by
taking it to the space described with the uniform velocity AB in a non¬
resisting medium, as the hyperbolic area ABGD to the rectangle AB X AD.

Cor. 3. The resistance of the medium is also given, by making it equal,
in the very beginning of the motion, to an uniform centripetal force, which
could generate, in a body falling through a non-resisting medium, the ve¬
locity AB in the time AC. For if BT be drawn touching the hyperbola
in B, and meeting the asymptote in T, the right line AT will be equal to
AC, and will express the time in which the first resistance, uniformly con
tinued, may take away the whole velocity AB

Cor. 4. And thence is also given the proportion of this resistance to the
force of gravity, or a~y other given centripetal force.

Cor. 5. And, vice versa , if there is given the proportion of the resist-
; nee to any given centripetal force, the time AC is also given, in which c
centripetal force equal to the resistance may generate any velocity as AB ;
and thence is given the poini B, through w T hich the hyperbola, having CH
CD for its asymptotes, is to be described : as also the space ABGD, which a
body, by beginning its motion with that velocity AB, can describe in any
time AD. in a similar resisting medium.

PROPOSITION VI. THEOREM IV.

Homogeneous and equal spherical bodies, opposed hy resistances that are
in the duplicate ratio of the velocities , and moving on by their innate
force only, will, in times which are reciprocally as the velocities at the

260 the mathematical principles [Book IL

A

beginning', describe equal spaces, and lose parts of their velocities pro¬
portional to the wholes.

To the rectangular asymptotes CD, CH de¬
scribe any hyperbola B6Ee, cutting the perpen¬
diculars AB, ab, DE, de in B, b, E, e; let the
initial velocities be expounded by the perpendicu¬
lars AB, DE, and the times by the lines A a, Drf.
Therefore as A a is to D d, so (by the hypothesis)
is DE to AB, and so (from the nature of the hy¬
perbola) is CA to CD ; and, by composition, so is
C a to C d. Therefore the areas AB ba, DE ed, that is, the spaces described,
are equal among themselves, and the first velocities AB, DE are propor¬
tional to the last ab, de; and therefore, by division, proportional to the
parts of the velocities lost, AB — ab, DE — de. Q.E.D.

PROPOSITION VII. THEOREM V.

If spherical bodies are resisted in, the duplicate ratio of their velocities ,
in times which are as the first motions directly, and the first resist -
ances inversely, they will lose parts of their mot ions proportional to the
wholes, and will describe spaces proportional to those times and the
first velocities conjunctly.

For the parts of the motions lost are as the resistances and times con¬
junctly. Therefore, that those parts may be proportional to the wholes,
the resistance and time conjunctly ought to be as the motion. Therefore the
time will be as the motion directly and the resistance inversely. Where¬
fore the particles of the times being taken in that ratio, the bodies will
always lose parts of their motions proportional to the wholes, and there¬
fore will retain velocities always proportional to their first velocities.
And because of the given ratio of the velocities, they will always describe
spaces which are as the first velocities and the times conjunctly. Q..E.D.

Cor. 1. Therefore if bodies equally swift are resisted in a duplicate ra¬
tio of their diameters, homogeneous globes moving with any velocities
whatsoever, by describing spaces proportional to their diameters, will lose
parts of their motions proportional to the wholes. For the motion of each
globe will be as its velocity and mass conjunctly, that is, as the velocity
and the cube of its diameter; the resistance (by supposition) will be as the
square of the diameter and the square of the velocity conjunctly; and the
•time (by this proposition) is in the former ratio directly, and in the latter
inversely, that is, as the diameter directly and the velocity inversely; and
therefore* the space, which is proportional to the time and velocity is as
the diameter.

Cor. 2. If bodies equally swift are resisted in a sesquiplicate ratio of
their diameters, homogeneous globes, moving with any velocities whatso-

Sec. IT.] of natural philosophy. 261

ever, by describing spaces that are in a sesquiplicate ratio of the diameters,
will lose parts of their motions proportional to the wholes.

Cor. 3. And universally, if equally swift bodies are resisted in the ratio
of any power of the diameters, the spaces, in which homogeneous globes,
moving with any velocity whatsoever, will lose parts of their motions pro¬
portional to the wholes, will be as the cubes of the diameters applied to
that power. Let those diameters be D and E; and if the resistances, where
the velocities are supposed equal, are as T) n and E n ; the spaces in which
the globes, moving with any velocities whatsoever, will lose parts of their
motions proportional to the wholes, will be as D 3 — n and E 3 — n . And
therefore homogeneous globes, in describing spaces proportional to D 3 — n
and E 3 — n , will retain their velocities in the same ratio to one another as
at the beginning.

Cor. 4. Now if the globes are not homogeneous, the space described by
the denser globe must be augmented in the ratio of the density. For the
motion, with an equal velocity, is greater in the ratio of the density, and
the time (by this Prop.) is augmented in the ratio of motion directly, and
the space described in the ratio of the time.

Cor. 5. And if the globes move in different mediums, the space, in a
medium which, cceteris paribus , resists the most, must be diminished in the
ratio of the greater resistance. For the time (by this Prop.) will be di¬
minished in the ratio of the augmented resistance, and the space in the ra¬
tio of the time.

LEMMA II.

The moment of any genitum is equal to the moments of each of the gen¬
erating sides drawn into the indices of the powers of those sides, and
into their co-efficients continually.

I call any quantity a genitum which is not made by addition or sub-
duetion of divers parts, but is generated or produced in arithmetic by the
multiplication, division, or extraction of the root of any terms whatsoever;
in geometry by the invention of contents and sides, or of the extremes and
means of proportionals. Quantities of this kind are products, quotients,
roots, rectangles, squares, cubes, square and cubic sides, and the like.
These quantities I here consider as variable and indetermined, and increas¬
ing or decreasing, as it were, by a perpetual motion or flux; and I under¬
stand their momentaneous increments or decrements by the name of mo¬
ments ; so that the increments may be esteemed as added or affirmative
moments; and the decrements as subducted or negative ones. But take
care not to look upon finite particles as such. Finite particles are not
moments, but the very quantities generated by the moments. We are to
conceive them as the just nascent principles of finite magnitudes. Nor do
we in this Lemma regard the magnitude of the moments, but their firsl

262

THE MATHEMATICAL PRINCIPLES

[Book 11

proportion, as nascent. It will be the same thing, if, instead of moments,
we use either the velocities of the increments and decrements (which may
also he called the motions, mutations, and fluxions of quantities), or any
finite quantities proportional to those velocities. The co-eflicient of any
generating side is the quantity which arises by applying the genitum to
that side.

Wherefore the sense of the Lemma is, that if the moments of any quan¬
tities A, B, C, (fee., increasing or decreasing by a perpetual flux, or the
velocities of the mutations which are proportional to them, be called a, 6,
c, (fee., the moment or mutation of the generated rectangle AB will be aB
4- bA; the moment of the generated content ABC will be aBC 4 bAC 4

_! j? 2

cAB; and the moments of the generated powers A 2 . A 3 , A 4 , A 2 , A 2 , A 3 ,

A 3 , A — A — 2 , A— 7 will be 2a A, 3aA 2 , 4aA 3 , \aA — 2 , faA*

2 1 _?

±aA — 5 , \a A — 3 , — a A — 2 , — 2aA — 3 , — \aA — 2 respectively; and

in general, that the moment of any power A~, will be ^ aA'^l^'. Also,

that the moment of the generated quantity A 2 B will be 2aAB 4- bA 2 ; the

moment of the generated quantity A 3 B 4 C 2 will be 3aA 2 B 4 C 2 +4AA 3

A 3

B 3 C 2 +2e-A 3 B 4 C; and the moment of the generated quantity jp or

A 3 B — 2 will be 3aA 2 B — 2 — 2bA 3 B — 3 ; and so on. The Lemma is
thus demonstrated.

Case 1. Any rectangle, as AB, augmented by a perpetual flux, when, as
yet, there wanted of the sides A and B half their moments \a and \b, was
A — \a into B — \b, or AB — B — \b A + \ab ; but as soon as the
sides A and B are augmented by the other half moments, the rectangle be¬
comes A + \a into B + \b, or AB + \a B + \b A 4 \ab. From this
rectangle subduct the former rectangle, and there will remain the excess
aB 4 bA. Therefore with the whole increments a and b of the sides, the
increment aB + bA of the rectangle is generated. Q.E.D.

Case 2. Suppose AB always equal to G, and then the moment of the
content ABC or GC (by Case 1) will be<gC + cG, that is (putting AB and
aB + bA for G and «*), aBC + bAC 4 cAB. And the reasoning is the
same for contents under ever so many sides. Q..E.D.

Case 3. Suppose the sides A, B, and C, to be always equal among them¬
selves; and the moment aB 4 bA, of A 2 , that is, of the rectangle AB,
will be 2aA ; and the moment aBC 4 bAC 4 cAB of A 3 , that is, of the
content ABC, will be 3aA 2 . And by the same reasoning the moment of
any power A n is naA n —\ Q.E.D

Case 4. Therefore since ~ into A is 1, the moment of ~ drawn into

Sec. 11.]

OF NATURAL PHILOSOPHY.

263

A, together with j drawn into a. will be the moment of 1, that is, nothing.

1 . — «

Therefore the moment of or of A— is . And generally since

t- into A n is 1, the moment of —drawn into A° together with — into
An ; A A n

?iaA n — 1 will be nothing. And, therefore, the moment of ^ or A — n

will be — . Q.E.D.

A n + 1

2.1. i i

Case 5. And since A 2 into A 2 is A, the moment of A^ drawn into 2A 3

i e a

will be a (by Case 3); and, therefore, the moment of A T will be or

{aA — And, generally, putting A^ 1 equal to B, then A m will be equal

to B n , and therefore maA m — 1 equal to nbB° — 1 , and maA — 1 equal to

?/Z>B — J , or nbA — ~; and therefore n a A ~~ is equal to b , that is, equal
to the moment of A^. Q.E.D.

Case 6. Therefore the moment of any generated quantity A m B n is the
moment of A m drawn into B n , together with the moment of B n drawn into
A‘“, that is, maA m — 1 B n + n6B n — 1 A ra ; and that whether the indices
m and n of the powers be whole numbers or fractions, affirmative or neg¬
ative. And the reasoning is the same for contents under more powers.
Q.E.D.

Cor. 1. Hence in quantities continually proportional, if one term is
given, the moments of the rest of the terms will be as the same terms mul¬
tiplied by the number of intervals between them and the given term. Let
A, B, C, D, E, F, be continually proportional; then if the term C is given,
the moments of the rest of the terms will be among themselves as — 2A,
— B, D, 2E, 3F.

Cor. 2. And if in four proportionals the two means are given, the mo¬
ments of the extremes will be as those extremes. The same is to be un¬
derstood of the sides of any given rectangle.

Cor. 3. And if the sum or difference of two squares is given, the mo¬
ments of the sides will be reciprocally as the sides.

SCHOLIUM.

In a letter of mine to Mr. J. Collins , dated December 10, 1672, having
described a method of tangents, which I suspected to be the same with
Slushis’s method, which at that time was not made public, I subjoined these
words • This is one particular , or rather a Corollary , of a general me

264

THE MATHEMATICAL PRINCIPLES

[Bjok IL

thod, which extends itself \ without any troublesome- calculation, not only
to the drawing of tangents to any curve lines, whether geometrical or
mechanical , or any how respecting right lines or other curves, but also
to the resolving other cibstruser kinds of problems about the crookedness ,
areas, lengths, centres of gravity of curves, &c.; nor is it (as Hudd'in’s
method de Maximis & Minimis) limited to equations which are free from
surd quantities. This method I have interwoven with that other oj
working in equations, by reducing them to infinite series. So far that
letter. And these last words relate to a treatise I composed on that sub¬
ject in the year 1671. The foundation of that general method is contain-,
od in the preceding Lemma.

PROPOSITION VIII. THEOREM VI.

mni

If a body in an uniform medium, being uniformly acted upon by the force
of gravity, ascends or descends in a right line ; and the whole space
described be distinguished into equal parts, and in the beginning of
each of the parts (by adding or subducting the resisting force of the
medium to or from the force of gravity , when the body ascends or de¬
scends) you collect the absolute forces ; I say, that those absolute forces
'ire in a geometrical progression.

fH ic For let the force of gravity be expounded by the

given line AC ; the force of resistance by the indefi¬
nite line AK ; the absolute force in the descent of the
Jff — ^ the difference KC; the velocity of the body

QPLK1A7X/ by a line AP, which shall be a mean proportional be¬
tween AK and AC, and therefore in a subduplicate ratio of the resistance;
the increment of the resistance made in a given particle of time by the li-
neola KL, and the contemporaneous increment of the velocity by the li-
neola PQ; and with the centre C, and rectangular asymptotes CA, CH,
describe any hyperbola BNS meeting the erected perpendiculars AB, KN,
liO in B, N and O. Because AK is as AP 2 , the moment KL of the one will
be as the moment 2APQ of the other, that is, as AP X KC ; for the in¬
crement PQ, of the velocity is (by Law II) proportional to the generating
force KC. Let the ratio of KL be compounded with the ratio KN, and
the rectangle KL X KN will become as AP X KC X KN ; that is (because
the rectangle KC X KN is given), as AP. But the ultimate ratio of the
hyperbolic area KNOL to the rectangle KL X KN becomes, when the
points K and L coincide, the ratio of equality. Therefore that hyperbolic
evanescent area is as AP. Therefore the whole hyperbolic area ABOL
is composed of particles KNOL which are always proportional to the
velocity AP; and therefore is itself proportional to the space described
with that velocity. Let .that area be now divided into equal parts

OF NATURAL PHILOSOPHY.

265

Sec. IJ.J

as ABMI, IMNK, KNOL, &c., and the absolute forces AC, IC, KC, LC,
&c., will be in a geometrical progression. Q,.E.D. And by a like rea¬
soning, in the ascent of the body, taking, on the contrary side of the point
A, the equal area's AB mi, imnk, kiwi, &c., it will appear that the absolute
forces AC. iG, kC, 1C, &c., are continually proportional. Therefore if all
the spaces in the ascent and descent are taken equal, all the absolute forces
1C, kC, iC, AC, IC, KC, LC, &c., will be continually proportional. Q,.E.D.

Cor. 1. Hence if the space described be expounded by the hyperbolic
area ABNK, the force of gravity, the velocity of the body, and the resist¬
ance of the medium, may be expounded by the lines AC, AP, and AK re¬
spectively ; and vice versa.

Cor. 2. And the greatest velocity which the body can ever acquire in
an infinite descent will be expounded by the line AC.

Cor. 3. Therefore if the resistance of the medium answering to any
given velocity be known, the greatest velocity will be found, by taking it
to that given velocity in a ratio subduplicate of the ratio which the force
of gravity bears to that known resistance of the medium.

PROPOSITION IX. THEOREM VII.

Supposing what is above demonstrated, I say, that if the tangents of the
angles of the sector of a circle, and of an hyperbola, be taken propor¬
tional to the velocit ies, the radius being of a jit magnitude, all the time
of the ascent to the highest place will be as the sector of the circle, and
all the tinve of descending from the highest place as the sector of the
hyperbola.

To the right line AC, which ex¬
presses the force of gravity, let AD be 5 ^
drawn perpendicular and equal. From
the centre D with the semi-diameter
AD describe as well the quadrant A/E
of a circle, as the rectangular hyper¬
bola AVZ, whose axis is AK, principal
vertex A, and asymptote DC. Let t)p,

DP be drawn; and the circular sector
A/D will be as all the time of the as¬
cent to the highest place ; and the hy¬
perbolic sector ATD as all the time of descent from the highest place; if
so be that the tangents Ap, AP of those sectors be as the velocities.

Case 1. Draw Dt’^ cutting off the moments or least particles tDv and
qT)p, described in the same time, of the sector AD/ and of the triangle
AD/?. Since those particles (because of the common angle D) are in a du-

qD p X /D
jt?D a

plicate ratio of the sides, the particle tDv will be

as

that is

266

THE MATHEMATICAL PRINCIPLES

[Book Ii.

(because /D is given), as

But jt?D 8 is AD 3 + Ap 2 , that is, AD 2 +
qT>p is } AD X pq . Therefore tDv, the

AD X Ale, or AD X Ok; and qDp is £ A D X pq. Therefore tDv, the

7}Q

particle of the sector, is as ^ ; that is, as the least decrement pq of the

velocity directly, and the force Ok which diminishes the velocity, inversely;
and therefore as the particle of time answering to the decrement of the ve¬
locity. And, by composition, the sum of all the particles tDv in the sector
AD/ will be as the sum of the particles of time answering to each of the
lost particles of the decreasing velocity Ap, till that velocity, being di¬
minished into nothing, vanishes; that is, the whole sector AD/ is as the
whole time of ascent to the highest place. Q.E.D.

Case 2. Draw DQV cutting off the least particles TDV and PDQ of
the sector DAY, and of the triangle DA Q ; and these particles will be to
each other as DT 2 to DP 2 , that is (if TX and AP are parallel), as DX 2
to DA 2 or TX 2 to AP 2 ; and, by division, as DX 2 — TX 2 to DA 2 —
AP 2 . But, from the nature of the hyperbola, DX 2 —TX 2 is AD 2 ; and, by
the supposition, AP 2 is AD X AK. Therefore the particles are to each
other as AD 2 to AD 2 —AD X AK ; that is, as AD to AD — AK or AC

to CK : and therefore the particle TDV of the sector is -—jYY—-', and

OK

PQ

therefore (because AC and AD are given) as ; that is, as the increment

of the velocity directly, and as the force generating the increment inverse¬
ly ; and therefore as the particle of the time answering to the increment.
And, by composition, the sum of the particles of time, in which all the par¬
ticles PQ of the velocity AP are generated, will be as the sum of the par¬
ticles of the sector ATI) ; that is, the whole time will be as the whole
sector. Q.E.D.

\ Cor. 1. Hence if AB be equal to a

Z \ fourth part of AC, the space which a body

\ \p will describe by falling in any time will

be to the space which the body could de-
\ scribe, by moving uniformly on in the

C\ same time with its greatest velocity

^\\\ //^\ AC, as the area ABNK, which es-

\^\\ // \ presses the space described in falling to

// \ the area ATD, which expresses the

time. For since AC is to AP as AP
to AK, then (by Cor. l,Lem. II, of this
Book) LK is to PQ as 2AK to AP, that is, as 2AP to AC, and thence
LK is to {PQ as AP to {AC or AB ; and KN is to AC or AD as AB U

OF NATURAL PHILOSOPHY.

267

JSec. II.]

UK; and therefore, ex ceqao, LKNO to DPQ, as AP to CK. But DPQ
was to DTV as CK to AC. Therefore, ex cequo , LKNO is to DTV r.s
AP to AC; that is, as the velocity of the falling body to the greatest
velocity which the body by falling can acquire. Since, therefore, the
moments LKNO and DTY of the areas ABNK and ATD are as the ve¬
locities, all the parts of those areas generated in the same time will be as
the spaces described in the same time ; and therefore the whole areas ABNK
and ADT, generated from the beginning, will be as the whole spaces de¬
scribed from the beginning of the descent. Q.E.D.

Cor. 2. The same is true also of the space described in the ascent.
That is to say, that all that space is to the space described in the same
time, with the uniform velocity AC, as the area ABnk is to the sector AD/.

Cor. 3. The velocity of the body, falling in the time ATD, is to the
velocity which it would acquire in the same time in a non-resisting space,
as the triangle APD to the hyperbolic sector ATD. For the velocity in
a non-resisting medium would be as the time ATD, and in a resisting me¬
dium is as AP, that is, as the triangle APD. And those velocities, at the
beginning of the descent, are equal among themselves, as well as those
areas ATD, APD.

Cor. 4. By the same argument, the velocity in the ascent is to the ve¬
locity with which the body in the same time, in a non-resisting space, would
lose all its motion of ascent, as the triangle ApD to the circular sector
A/D; or as the right line A p to the arc At.

Cor. 5. Therefore the time in which a body, by falling in a resisting
medium, would acquire the velocity AP, is to the time in which it would
acquire its greatest velocity AC, by falling in a non-resisting space, as the
sector ADT to the triangle ADC: and the time in which it would lose its
velocity A p, by ascending in a resisting medium, is to the time in which
it would lose the same velocity by ascending in a non-resisting space, as
the arc At to its tangent Ap.

Cor. 6. Hence from the given time there is given the space described in
the/ascent or descent. For the greatest velocity of a body descending in
infinitum is given (by Corol. 2 and 3, Theor. VI, of this Book); and thence
the time is given in which a body would acquire that velocity by falling
in a non-resisting space. And taking the sector ADT or AD/ to the tri¬
angle ADC in tbe ratio of the given time to the time just now found,
there will be given both the velocity AP or A p, and the area ABNK or
AB//A;, which is to the sector ADT, or AD/, as the space sought to the
space which would, in the given time, be uniformly described with that
greatest velocity found just before.

Cor. 7. And by going backward, from the given space of ascent or de¬
scent AB nk or ABNK, there will be given the time AD/ or ADT.

268

THE MATHEMATICAL PRINCIPLES

[Book II

PROPOSITION X. PROBLEM III.

Suppose the uniform force of gravity to tend directly to the plane of the
horizon , and the resistance to be as the density of the medium and the
square of the velocity conjuuctly : it is proposed to find the density of
the medium in each place , ivhich shcdl make the body move in any
given curve line ; the velocity of the body and the resistance of the
medium in each place.

Let PQ be a plane perpendicular to
the plane of the scheme itself; PFHQ
a curve line meeting that plane in the
points P and Q; G, H, I, K four
places of the body going on in this
curve from F to Q; and GB ; HO, ID,
KE four parallel ordinates let fall
P a. b c d e q from these points to the horizon, and

standing on the horizontal line PQ, at the points B, C, D, E; and let the
distances BC, CD, DE, of the ordinates be equal among themselves. From
the points G and H let the right lines GL, HN, be drawn touching the
curve in G and H, and meeting the ordinates CH, DI, produced upwards,
in L and N : and complete the parallelogram HO DM. And the times in
which the body describes the arcs GH, HI, will be in a subduplicate ratio
of the altitudes LH, NI, which the bodies would describe in those times,
by falling from the tangents; and the velocities will be as the lengths de¬
scribed GH, HI directly, and the times inversely. Let the times be ex-

GH HI

pounded by T and t , and the velocities by -jr and —- ; and the decrement

GH HI

of the velocity produced in the time t will be expounded by -pjv-—.

This decrement arises from the resistance which retards the body, and from
the gravity which accelerates it. Gravity, in a falling body, which in its
fall describes the space NI, produces a velocity with which it would be able
to describe twice that space in the same time, as Galileo has demonstrated ;
2NI

that is, the velocity : but if the body describes the arc HI, it augments

MI X NI

that arc only by the length HI — HN or —gj—; and therefore generates
2M1 X NI

only the velocity —-7x7“* I*et this velocity be added to the before-

l /\ III

mentioned decrement, and we shall have the decrement of the velocity

. GH HI 2MI X NI

arising from the resistance alone, that is, "TjT” T + Tx HI '

Sec. II.]

OF NATURAL PHILOSOPHY.

269

Therefore since, in the same time, the action of gravity generates, in afall-

2NI GH

ing body, the velocity ——, the resistance will be to the gravity as —--

t JL

HI
t

2MI X NI 2NI t X GH
+ TxThT t0 ~T or M T

2MI X NI

+ -7T7— tc 2NI.

Now for the abscissas CB, CD,

OE, put — o, o, 2o. For the ordinate
CH put P j and for MI put any series
Qo + Ro 2 + So 3 +, <fcc. And all
the terms of the series after the lirst,
that is, Ro 2 -f So 3 +, (fee., will be
NI; and the ordinates DI, EK, and

BG will be P — Qo — Ro 2 — So 3 —, p A. B c D e q

(fee., P_2Qo —4Ro 2 —SSo 3 —, (fee., and P -f- Qo —Ro 2 + So 3 —,
(fee., respectively. And by squaring the differences of the ordinates BG —
CH and CH — DI, and to the squares thence produced adding the squares
of BC and CD themselves, you will have oo + QQoo — 2QRo 3 +, (fee.,
and oo + QQoo + 2QRo 3 +, (fee., the squares of the arcs GH, HI; whose

QRoo_QRoo

roots o y/ --, and o y/\ i qq _l_ —- are the

1 + QQ v/l+QQ +HH+ v/1+QQ

arcs GH and HI. Moreover, if from the ordinate CH there be subducted
half the sum of the ordinates BG and DI, and from the ordinate DI there
be subducted half the sum of the ordinates CH and EK, there will remain
Roo and Roo + 3So 3 , the versed sines of the arcs GI and HK. And these
are proportional to the lineolae LH and NI, and therefore in the duplicate

ratio of the infinitely small times T and t: and thence the ratio ~ is y/

R + 3So R 4- #So
or

R

the values of

R

t_

T

and

t X GH

HI +

2MI X NI

T 1 HI

GH, HI, MI and NI just found, becomes

, by substituting

3Soo
~2R

v/

l + QQ. And since 2NI is 2Roo, the resistance will be now to the

3Soo _ _

gravity as qq to 2Roo, that is, as 3S + qq to 4RR.

And the velocity will be such, that a body going off therewith from any
place H, in the direction of the tangent HN, would describe, in vacuo, a

parabola, whose diameter is HC, and its latus rectum or —

And the resistance is as the density of the medium and the square of
the velocity conjunctly ; and therefore the density of the medium is as the
resistance directly, and the square of the velocity inversely; that is, as

270

THE MATHEMATICAL PRINCIPLES

[Book II.

3S vl

directly and _

1 + QQ, •

inversely; that is, as

4RR R R V 1 + OO

O.E.I.

Cor. 1. If the tangent HN be produced both ways, so as to meet any

jjrp

ordinate AF in T will be equal to X -f qq, an 4 therefore in what

has gone before may be put for v 1 -f OO. By this means the resistance
will be to the gravity as 3S X HT to 4RR X AC; the velocity will be a *

ttt— 7 it, and the density of the medium will be as

AC V R J R X HT

Cor. 2. And hence, if the curve line PFHO be defined by the relation
between the base or abscissa AC and the ordinate CH ; as is usual, and the
value of the ordinate be resolved into a converging series, the Problem
will be expeditiously solved by the first terms of the series; as in the fol¬
lowing examples.

Example 1. Let the line PFHO be a semi-circle described upon the
diameter PO, to find the density of the medium that shall make a projec¬
tile move in that line.

Bisect the diameter PO in A ; and call AO, n ; AC, a ; CH, e ; and
CD, o; then DI 2 or AO 2 — AD 2 = nn — aa — 2ao — oo, or eu — 2ao
— oo ; and the root being extracted by our method, will give DI = e —
ao oo aaoo an* a* o*

—, (fee.

Here put nn for ee

aa, and

. aO 717100

DI will become = e -——

e 2e 3

anno 3

—> &c -

Such series I distinguish into successive terms after this manner: I call
that the first term in which the infinitely small quantity o is not found;
the second, in which that quantity is of one dimension only; the third, in
which it arises to two dimensions; the fourth, in which it is of three; and
so ad infinitum. And the first term, which here is e, will always denote
the length of the ordinate CH, standing at the beginning of the indefinite

quantity o. The second term, which here is will denote the difference

between CH and DN; that is, the lineola MN which is cut off by com¬
pleting the parallelogram HCDM; and therefore always determines the

cto

position of the tangent HN; as, in this case, by taking MN to HM as —

to o, or a to e. The third term, which here is "gJT? will represent the li¬
neola IN, which lies between the tangent and the curve; and therefore
determines the angle of contact IHN, or the curvature which the curve line

OF NATURAL PHILOSOPHY.

271

Sec. II.]

has in H. If that lineola IN is of a finite magnitude, it will be expressed
by the third term, together with those that follow in infinitum. Hut if
that lineola be diminished in infini¬
tum, the terms following become in¬
finitely less than the third term, and
therefore may be neglected. The
fourth term determines the variation
of the curvature; the fifth, the varia¬
tion of the variation ; and so on.

Whence, by the way, appears no con- p a b c d e q.

temptible use of these series in the solution of problems that depend upon
tangents, and the curvature of curves.

ao

Now compare the series e — —

nnoo

2e 3

anno 3
~2e*~

— &c., with the

series P — 0,0 -- Ron — So 3

(fee., and for P, Q, R and S, put e,

G *£>G

and and for 1 + QQ put ^ 1 + — or -; and the density of

s GG 6

a . . a

the medium will come out as —; that is (because n is given), as - or

that is, as that length of the tangent HT, which is terminated at the

semi-diameter AF standing perpendicularly on PO: and the resistance
will be to the gravity as 3 a to 2n, that is, as 3AC to the diameter PO of
the circle; and the velocity will be as ^CH. Therefore if the body goes
from the place F, with a due velocity, in the direction of a line parallel to
PO, and the density of the medium in each of the places II is as the length
of the tangent HT, and the resistance also in any place H is to the force
of gravity as 3AC to PO, that body will describe the quadrant FHO of a
circle. O.E.I.

But if the same body should go*from the place P, in the direction of a
line perpendicular to PO, and should begin to move in an arc of the semi¬
circle PFO, we must take AC or a on the contrary side of the centre A ;
and therefore its sign must be changed, and we must put — a for -f a.

ft

Then the density of the medium would come out as-. But nature

6

does not admit of a negative density, that is, a density which accelerates
the motion of bodies; and therefore it cannot naturally come to pass that
a body by ascending from P should describe the quadrant PF of a circle.
To produce such an effect, a body ought to be accelerated by an impelling
medium, and not impeded by a resisting one.

Example 2. Let the line PFQ, be a parabola, having its axis AF per-

272

THE MATHEMATICAL PRINCIPLES

[Book BL

pendicular to the horizon PQ, to find the density of the medium, which
will make a projectile move in that line.

v -g- From the nature of the parabola, the rectangle PDQ,

1 is equal to the rectangle under the ordinate DI and some
given right line ; that is, if that right line be called b ;
PC, a; PQ,, c; CH, e; and CD, o; the rectangle a
A. CD Q + o into c — a — o or ac — aa — 2ao -j- co — oo, is

ac — aa

equal to the rectangle b into DI, and therefore DI is equal to-^-h

c — 2a oo , c — 2a .

o -r. Now the second term —— o of this series is to he put

oo

b ~ b

for Q,o, and the third term — for Roo. But since there are no more
terms, the co-efficient S of the fourth term will vanish ; and therefore the

quantity

R v x 1 + OO

, to which the density of the medium is propor¬

tional, will be nothing. Therefore, where the medium is of no density,
the projectile will move in a parabola; as Galileo hath heretofore demon¬
strated. O.E.I.

Example 3. Let the line AGK be an hyperbola, having its asymptote
NX perpendicular to the horizontal plane AK, to find the density of the
medium that will make a projectile move in that line.

Let MX be the other asymptote, meeting
the ordinate DG produced in Y; and from

XY into YG will be given. There is also
given the ratio of DN to YX, and therefore
the rectangle of DN into YG is given. Let
that be bb: and, completing the parallelo¬
gram DNXZ, let BN be called a; BD, o ;
NX, c; and let the given ratio of YZ to

m

ZX or DN be —. Then DN will be equal
n

m

m

m,

n

bb

bb

be

-YZ—YG equal to c- a -1- o -. Let the term --

^ n n a—o a — o

. . bb bb bb bb

resolved into the converging series-1- 0 + -^oo + -— o 3 , &c., and

(jl act ci ci

GD will become equal to c

m

bb m

- 1 - o

a n

bb

bb

bb

aa

Sec. II.]

OF NATURAL PHILOSOPHY.

273

&c. The second term — o — — o of this series is to be used for do; the
n aa

third ^ o 2 , with its sign changed for Ro 2 ; and the fourth ~ o 3 , with its

m bb bb bb

sign changed also for So 3 , and their coefficients-, — and — are to

° ° ’ n aa a 3 a 4

be put for Q,, R, and S in the former rule. Which being done, the den-

bb

sity of the medium will come out as

~/x +

mm

nn

2mbb b*

-1--or

naa a 4

>/ mm

aa H- aa

nn

1

b 4

2mbb
n aa
m 2

\ that is, if in YZ you take VY equal to

2mbb b 4

H-are the squares of XZ

YG, as For aa and

’ XY n J n aa

and ZY. But the ratio of the resistance to gravity is found to be that of

3XY to 2YG; and the velocity is that with which the body would de-

XY 2

scribe a parabola, whose vertex is G, diameter DG, latus rectum “yQ - * Sup¬

pose, therefore, that the densities of the medium in each of the places G
are reciprocally as the distances XY, and that the resistance in any place
G is to the gravity as 3XY to 2YG ; and a body let go from the place A,
with a due velocity. will describe that hyperbola AGK. Q.E.I.

Example 4. Suppose, indeSnitely, the line AGK to be an hyperbola
described with the centre X, and the asymptotes MX, NX, so that, having
constructed the rectangle XZDN, whose side ZD cuts the hyperbola in G
and its asymptote in Y, YG may be reciprocally as any power DN n of the
line ZX or DN, whose index is the number n: to find the density of the
medium in which a projected body will describe this curve.

For BN, BD, NX, put A, O, C, respec¬
tively, and let YZ be to XZ or DN as d to

e, arid VG be equal to ; then DN will
be equal to A — O, VG = - ■ — VZ =

- A^o", and GD or NX — VZ — VG equal
d d bb

to O-AH-O — Let the

e e A — 0|° jjt

274

THE MATHEMATICAL PRINCIPLES

[Book II

bb . . . bb nbb _

term __ _ n be resolved into an infinite senes -r^ + —-x O +

A — 0| A 1 A. n + 1

nn + n _, ^ n 3 + 3 ? 7?7 + 271 ,, _

2 A" +~ » * ^ O 2 H- ^ - n 3 - X bb O 3 , &c., and GD will be equal

nbb

„ d bb <Z

t° c — - a - ^ + -°- a „ + ,

+ 7i 3 + 3nn +2*7

»• -

6A n +

bbO 3 , &c. The second term - O — n ™-- - O of this

e A n + 1

series is to be used for Qo, the third a bbO 2 for Roo, the fourth

77 3 + 3/777 + 2/7

— g]Y" - + - 3- bbO 3 for So 3 . And thence the density of the medium

H v/ l + QQ

, in anyplace G, will be

n + 2

3 v/ , dd 2dnbb ////o 4 ‘

A, + ^ A2 -1a^ A + X-

and therefore if in YZ you take VY equal to n X YG, that density is re-

n vv t? 10 1 dd 2 dnbb nnb 4

ciprocally as Xi. For A- and — A 2 — — 7 — A + -— are the

ee eA u A 2n

squares of XZ and ZY. But the resistance in the same place G is to the

XY 4- 2n

force of gravity as 3S X - 7 — to 4RR, that is, as XY to —-- YG.

A 77+2

And the velocity there is the same wherewith the projected body would

move in a parabola, whose vertex is G, diameter GD, and latus rectum

1+GQ, 2XY 2

or =-. Q.E.I.

R

nn + 77 X VG

SCHOLIUM.

In the same manner that the den¬
sity of the medium comes out to be aa
S X AC . ^

R ~ X ~ H T’ m (> ° r * resistanoe

is put as any power V n of the velocity
V, the density of the medium will

come out to be as

4 — n

R-r-

And therefore if a curve can be found, such that the ratio of —-— to

' 4 — n

R 2~

Sec. II.J

OF NATURAL PHILOSOPHY.

275

HT

AC

n — I

or of

S 2

R 4 -

to

i + aa\ n

may be given; the body, in an

uni¬

form medium, whose resistance is as the power V n of the velocity V, will
move in this curve. But let us return to more simple curves.

Because there can be no motion in a para¬
bola except in a non-resisting medium, but
in the hyperbolas here described it is produced
by a perpetual resistance; it is evident that
the line which a projectile describes in an
uniformly resisting medium approaches nearer
to these hyperbolas than to a parabola. That
line is certainly of the hyperbolic kind, but
about the vertex it is more distant from the
asymptotes, and in the parts remote from the

vertex draws nearer to them than these hy- MT”3. BD~KT N

perbolas here described. The difference, however, is not so great between
the one and the other but that these latter may be commodfously enough
used in practice instead of the former. And perhaps these may prove more
useful than an hyperbola that is.more accurate, and at the same time more
compounded. They may be made use of, then, in this manner.

Complete the parallelogram XYGT, and the right line GT will touch
the hyperbola in G, and therefore the density of the medium in G is re-

GT 2

ciprocally as the tangent GT, and the velocity there as

and the

resistance is to the force of gravity as GT to

Therefore if a body projected from the
place A, in the direction of the right line
AH, describes the hyperbola AGK and
AH produced meets the asymptote NX in
H, and AI drawn parallel to it meets the
other asymptote MX in I; the density of
the medium in A will be reciprocally as
AH. and the velocity of the body as V
AH*

■ , and the resistance there to the force

AI

2 nn + 2n

ii + 2

X GY.

of gravity r.s

AH t 2nn + 2n

AH to-

n + 2

X AI. Hence the

following

rules a e

deduced.

Rule 1. If the density of the medium at A, and the velocity with which
the body is projected remain the same, and the angle NAH be changed,
the lengths AH, AI, HX will remain. Therefore if those lengths, in any

276 THE MATHEMATICAL PRINCIPLES [BOOK II.

one case, are found, the hyperbola may afterwards be easily determined
from any given angle NAH.

Rule 2. If the angle NAH, and the density of the medium at A, re¬
main the same, and the velocity with which the body is projected be
changed, the length AH will continue the same ; and AI will be changed
in a duplicate ratio of the velocity reciprocally.

Rule 3. If the angle NAH, the velocity of the body at A, and the ac¬
celerative gravity remain the same, and the proportion of the resistance at
A to the motive gravity be augmented in any ratio; the proportion of AH
to AI will be augmented in the same ratio, the latus rectum of the above-

AH 2

mentioned parabola remaining the same, and also the length propor-

Al

tional to it; and therefore AH will be diminished in the same ratio, and
AI will be diminished in the duplicate of that ratio. But the proportion
of the resistance to the weight is augmented, when either the specific grav-
ity is made less, the magnitude remaining equal, or when the density of
the medium is made greater, or when, by diminishing the magnitude, the
resistance becomes diminished in a less ratio than the weight.

Rule 4. Because the density of the medium is greater near the vertex
of the hyperbola than it is in the place A, that a mean density may be
preserved, the ratio of the least of the tangents GT to the tangent AH
ought to be found, and the density in A augmented in a ratio a little
greater than that of half the sum of those tangents to the least of the
tangents GT.

Rule 5. If the lengths AH, ,AI are given, and the figure AGK is to be
described, produce HN to X, so that HX may be to AI as n -f 1 to 1; and
with the centre X, and the asymptotes MX, NX, describe an hyperbola
through the point A, such that AI may be to any of the lines YG as XV"
to XI".

Rule 6 . By how much the greater the number n is, so much the more
accurate are these hyperbolas in the ascent of the body from A, and less
accurate in its descent to K; and the contrary. The conic hyperbola
keeps a mean ratio between these, and is more simple than the rest. There¬
fore if the hyperbola be of this kind, and you are to find the point K,
where the projected body falls upon any right line AN passing through
the point A, let AN produced meet the asymptotes MX, NX in M and N,
and take NK equal to AM.

Rule 7. And hence appears an expeditious method of determining this
hyperbola from the phenomena. Let two similar and equal bodies be pro¬
jected with the same velocity, in different angles HAK, hAk , and let them
fall upon the plane of the horizon in K and k ; and note the proportion f
of AK to Ak. Let it be as d to e. Then erecting a perpendicular AI of
any length, assume any how the length AH or Ah, and thence graphically,

Sec. II.]

OF NATURAL PHILOSOPHY.

2 77

or by scale and compass, collect the lengths AK, Ak (by Rule 6). If the
ratio of AK to Ak be the same with that of d to e, the length of AH was

rightly assumed. If not, take on the indefinite right line SM, the length
SM equal to the assumed AH; and erect a perpendicular MN equal to the

difference — - of the ratios drawn into any given right line. By the

like method, from several assumed lengths AH, you may find several points
N ; and draw througli them all a regular curve NNXN, cutting the right
line SMMM in X. Lastly, assume AH equal to the abscissa SX, and
thence find again the length AK; and the lengths, which are to the as¬
sumed length AI, and this last AH, as the length AK known by experi¬
ment, to the length AK last found, will be the true lengths AI and AH,
which were to be found. But these being given, there will be given also
the resisting force of the medium in the place A, it being to the force of
gravity as AH to £AI. Let the density of the medium be increased by
Rule 4, and if the resisting force just found be increased in the same ratio,
it will become still more accurate.

Rule S. The lengths AH, HX being found ; let there be now re¬
quired the position of the line AH, according to which a projectile thrown
with that given velocity shall fall upon any point K. At the [joints A
and K, erect the lines AC, KF perpendicular to the horizon ; whereof let
AC be drawn downwards, and be equal to AI or ^HX. With the asymp¬
totes AK, KF, describe an hyperbola, whose conjugate shall pass through
the point C ; and from the centre A, with the interval AH. describe a cir¬
cle cutting that hyperbola in the point H; then the projectile thrown in
the direction of the right line AH will fall upon the point K. Q.E.I. For
the point H, because of the given length AH, must be somewhere in the
circumference of the described circle. Draw CH meeting AK and KF in
E and F; and because CH, MX are parallel, and AC, AI equal, AE will
be equal to AM, and therefore also equal to KN. But CE is to AE as
FH to KN, and therefore CE and FH are equal. Therefore the point H
falls upon the hyperbolic curve described with the asymptotes AK,.KF
whose conjugate passes through the point C ; and is therefore found in the

27 S

THE MATHEMATICAL PRINCIPLES

[Book 1L

common intersection of this hyperbolic
curve and the circumference of the de-
/ I scribed circle. Q.E.D. It is to be oh

sy j served that this operation is the same,

x' '■ \\ whether the right line AKN be parallel to

Jx; the horizon, or inclined thereto in any an-

’ an< ^ ^ rom ^ w0 i n t ersec tions H,

A _--Vi " t \ K there arise two angles NAH, NAA ;

/. ^ \_ _ and that in mechanical practice it is suf-

M c!/ 6 AK N ficient once to describe a circle, then to

apply a ruler CH, of an indeterminate length, so to the point C, that its
part PH, intercepted between the circle and the right line FK, may be
equal to its part CE placed between the point C and the right line AK

What has been said of hyperbolas may he easily
T Jv applied to p ir ibid h. For if a parabola be re-

/ presented by XAGK, touched by a right line XV

\v in the vertex X, and the ordinates I A, VG be as

any powers XI n , XV“, of the abscissas XI, XV;
^ draw XT, GT, AH, whereof let XT be parallel

\ to VG, and let GT, AH touch the parabola in
B Y G and A : and a body projected from any place

^ A, in the direction of the right line AH, with a

due velocity, will describe this parabola, if the density of the medium in
each of the places G be reciprocally as the tangent GT. In that case the
velocity in G will be the same as would cause a body, moving in a non¬
resisting space, to describe a conic parabola, having G for its vertex, VG

2GT 2

produced downwards for its diameter, and -— _- for its latus

nn — n X VG

rectum. And the resisting force in G will be to the force of gravity as GT to
2 nn — 2it .

• " o — * G. Therefore if NAK represent an horizontal line, and botli

the density of the medium at A, and the velocity with which the body is
projected, remaining the same, the angle NAH be any how altered, the
lengths AH, AI, HX will remain; and thence will be given the vertex X
of the parabola, and the position of the right line XI; and by taking VG
to IA as XV n to X l n , there will be given all the points G of the parabola,
through which the projectile will pass.

Sec. III.]

OF NATURAL PHILOSOPHY.

279

SECTION III.

Of the motions of bodies which are resisted partly In the ratio of the ve¬
locities, and partly in the duplicate of the same rat io.

|H

\B

Gr j\_

D

V

PROPOSITION XI. THEOREM VIII.

If a body be resisted partly in the ratio and partly in the duplicate rat io
of its velocity , and moves in a similar medium by its innate force
only; and the times be taken in arithmetical progression; then
quantities reciprocally proportional to the velocities, increased by a cer¬
tain given quantity , will be in geometrical progression.

With the centre C, and the rectangular asymptotes
CAM and CH, describe an hyperbola BEe, and let
AB, DE, de , be parallel to the asymptote CH. In
the asymptote CD let A, G be given points ; and if
the time be expounded by the hyperbolic area ABED
uniformly increasing, I say, that the velocity may
be expressed by the length DF, whose reciprocal
GD, together with the given line CG, compose the
length CD increasing in a geometrical progression.

For let the areola DE ed be the least given increment of the time, and
Dd will be reciprocally as DE, and therefore directly as CD. Therefore

the decrement of which (by Lem. II, Book II) is will be also as

CD CG + GD 1 CG

?TTTror —ttfTo - > that is, as ttf: + tttt«• Therefore the time ABED
GL)“ OIJ GLH

uniformly increasing by the addition of the given particles EDc/e, it fol¬
lows that decreases in the same ratio with the velocity. For the de¬
crement of the velocity is as the resistance, that is (by the supposition), as
the sum of two quantities, whereof one is as the velocity, and the other as

the square of the velocity ; and the decrement of is as the sum of the

1

CG

1

quantities and whereof the first is „„

GL> uL)“ GJJ

itself, and the last

CG 1 1

^pr -isas : therefore 7 ^ is as the velocity, the decrements of both
GD 2 GD 2 GD

being analogous. And if the quantity GD reciprocally proportional to

—be augmented by the given quantity CG; the sum CD, the time

ABED uniformly increasing, will increase .'n
Q.E.D.

geometrical progression.

THE MATHEMATICAL PRINCIPLES

280

[Book II

Cor. 1. Therefore, if, haying the points A and G given, the time be
expounded by the hyperbolic area ABED, the velocity may be expounded

by the reciprocal of GD.

Cor. 2. And by taking GA to GD as the reciprocal of the velocity at
the beginning to the reciprocal of the velocity at the end of any time
ABED, the point G will be found. And that point being found the ve¬
locity may be found from any other time given.

C G A.

T>c£

PROPOSITION XII. THEOREM IX.

The same things being supposed , I say, that if the spaces described are.
taken in arithmetical progression, the velocities augmented by a cer -
tain given quantity will be in geometrical progression.

K| In the asymptote CD let there be given the

\j 3 point R, and, erecting the perpendicular R§

meeting the hyperbola in S, let the space de-
*2E „ scribed be expounded by the hyperbolic area

RSED ; and the velocity will be as the length
GD, which^'together with the given line CG,
composes a length CD decreasing in a geo¬
metrical progression, while the space RSED increases in an arithmetical
progression.

For, because the incre nent ED de of the space is given, the lineola D d,
which is the decrement of GD, will be reciprocally as ED, and therefore
directly as CD ; that is, as the sum of the same GD and the given length
CG. But the decrement of the velocity, in a time reciprocally propor¬
tional thereto, in which the given particle of space Dt/eE is described, is
as the resistance and the time conjunctly, that is, directly as the sum ot
two quantities, whereof one is as the velocity, the other as the square of
the velocity, and inversely as the veh city; and therefore directly as the
sum of two quantities, one of which is given, the other is - as the velocity.
Therefore the decrement both of the velocity and the line GD is as a given
quantity and a decreasing quantity conjunctly; and, because the decre¬
ments are analogous, the decreasing quantities will always be analogous;
viz., the velocity, and the line GD. Q.E.D.

Cor. 1. If the velocity be expounded by the length GD, the space de¬
scribed will be as the hyperbolic area DE8R.

Cor. 2. And if the point . be assumed any how, the point G will be
found, by taking GR to GD as the velocity at the beginning to the velo¬
city after any space RSED is described. The point G being given, the
space is given from the given velocity: and the contrary.

Co«. 3. Whence since (by Prop. XI) the velocity is given from the given

Sec. Ill.!

or NATURAL PHILOSOPHY.

2S1

time, and (by this Prop.) the space is given from the given velocity; the
space will be given from the given time : and the contrary.

PROPOSITION NIII. THEOREM X.

Supposing that a body attracted downwards by an uniform gravity as¬
cends or descends in a right line; and that the same is resisted
partly in the ratio of its velocity, and partly in the duplicate ratio
thereof: I say, that, if right lines parallel to the diameters of a circle
and an hyperbola- be drawn through the ends of the conjugate diame¬
ters, and the velocities be as some segments of those parallels drawn
from a given point, the times will be as the sectors of the areas cut
off by right lines drawn from the centre to the ends of the segments;
and the contrary.

Case 1 . Suppose first that the body is ascending,
and from the centre I), with any semi-diameter DB,
describe a quadrant BETF of a circle, and through
the end B of the semi-diameter DB draw the indefi¬
nite line BAP, parallel to the semi-diameter DF. In
chat line let there be given the point A, and take the
Begment AP proportional to the velocity. And since
one part of the resistance is as the velocity, and
another part as the square of the velocity, let the
whole resistance be as AP 2 4- 2BAP. Join DA, DP, cutting the circle
in E and T, and let the gravity be expounded by DA 2 , so that the gravity
shall be to the resistance in P as DA 2 to AP 2 -f2BAP; and the time of the
whole ascent will be as the sector EDT of the circle.

For draw DVQ, cutting off the moment PQ of the velocity AP, and the
moment DTV of the sector DET answering to a given moment of time ;
and that decrement PQ, of the velocity will be as the sum of the forces of
gravity DA 2 and of resistance AP 2 + 2BAP, that is (by Prop. XII
Book II,Elem.),as DP 2 . Then the arsa DPQ, which is proportional to PQ,
is as DP 2 , and the area DTV, which is to the area DPQ as DT 2 to DP 2 , is
as the given quantity DT 2 . Therefore the area EDT decreases uniformly
according to the rate of the future time, by subduction of given particles DT V 7 ,
and is therefore proportional to the time of the whole ascent. Q.E.D.

Case 2. If the velocity in the ascent
of the body be expounded by the length Q
AP as before, and the resistance be made
as AP 2 4- 2BAP, and if the force of grav¬
ity be less than can be expressed by DA 2 ;
take BD of such a length, that AB 2 —

BD 2 may be proportional to the gravity,
and let DF be perpendicular and equal D

282

THE MATHEMATICAL PRINCIPLES

[Book II.

tro DB, and through the vertex F describe the hyperbola FTVE, whose con¬
jugate semi-diameters are DB and DF, and which cuts DA in E, and DP,
DQ in T and V; and the time of the whole ascent will be as the hyper¬
bolic sector TDE.

For the decrement PQ of the velocity, produced in a given particle of
time, is as the sum of the resistance AP 2 -f 2BAP and of the gravity
AB 2 — BD 2 , that is, as BP 2 — BD 2 . But the area DTY is to the area
DPQ as DT 2 to DP 2 ; and, therefore, if GT be drawn perpendicular to
DF. as GT 2 or GD 2 — DF 2 to BD 2 , and as GD 2 to BP 2 , and, by di¬
vision, as DF 2 to BP 2 — BD 2 . Therefore since the area DPQ, is as PQ,
that is, as BP 2 — BD 2 , the area DTY will be as the given quantity DF 2 .
'Therefore the area EDT decreases uniformly in each of the equal particles
of time, by the subduction of so many given particles DTY, and therefore
is proportional to the time. Q.E.D.

r CASE 3. Let AP be the velocity in the descent of
the body, and AP 2 + 2BAP the force of resistance,
andBD 2 —AB 2 the force of gravity, the angle DBA
being a right one. And if with the centre D, and the
principal vertex B, there be described a rectangular
hyperbola BETY cutting DA, DP, and DQ produced
in E, T, and V ; the sector DET of this hyperbola will
be as the whole time of descent.

For the increment PQ of the velocity, and the area DPQ proportional
to it, is as the excess of the gravity above the resistance, that is, as

|» 1)2 AB 2 2BAP — AP 2 or BD 2 — BP 2 . And the area DTV

is to the area DPQ as DT 2 to DP 2 ; and therefore as GT 2 or GD 2 —
BD 2 to BP 2 , and as GD 2 to BD 2 , and, by division, as BD 2 to BD 2 —
BP 2 . Therefore since the area DPQ is as BD 2 — BP 2 , the area DTV
will be as the given quantity BD 2 . Therefore the area EDT increases
uniformlv in the several equal particles of time by the addition of as
many given particles DTY, and therefore is proportional to the time of
the descent. Q.E.D.

Cor. If with the centre D and the semi-diameter DA there be drawn
through the vertex A an arc At similar to the arc ET, and similarly sub-
tendin^the angle A DT, the velocity AP will be to the velocity which the
body in the time EDT, in a non-resisting space, can lose in its ascent, or
acquire in its descent, as the area of the triangle DAP to the area of the
Bector DA£ ; and therefore is given from the time given. For the velocity
in a non-resisting medium is proportional to the time, and therefore to this
sector: in a resisting medium, it is as the triangle; and in both mediums,
where it is least, it approaches to the ratio of equality, as the sector and
triangle do

Sec. III.]

OF NATURAL PHILOSOPHY.

283

SCHOLIUM.

One may demonstrate also that case in the ascent of the body, where the
force of gravity is less than can be expressed by DA 2 or AB 2 + BD 2 , and
greater than can be expressed by AB 2 — DB 2 , and must be expressed by
AB 2 . But I hasten to other things.

PROPOSITION XIV. THEOREM XI.

The same things being supposed , 1 say , that the space described in the
ascent or descent is as the difference of the area by which the time is
expressed , and of some other area which is augmented or diminished
in an arithmetical progression ; if the forces compounded of the re¬
sistance and the gravity be taken in a geometrical progression.

Take AC (in these three figures) proportional to the gravity, and AK
to the resistance; but take them on the same side of the point A, if the

body is descending, otherwise on the contrary. Erect A b, which make to
DB as DB 2 to 4BAC : and to the rectangular asymptotes CK, CH, de¬
scribe the hyperbola 6N: and, erecting KN perpendicular to CK, the area
A/;NK will be augmented or diminished in an arithmetical progression,
while the forces CK are taken in a geometrical progression. I say, there¬
fore, that the distance of the body from its greatest altitude is as the excess
of the area A6NK above the area DET.

For since AK is as the resistance, that is, as AP 2 X 2BAP; assume

any given quantity Z, and put AK equal to

2BAP

; then (by Lem.

284

THE MATHEMATICAL PRINCIPLES [BOOK II

II of this Book) the moment KL of AK will be equal to

2APQ + 2BA X PQ
Z

or —~—, and the moment KLON of the area A6NK will be equal to

2BPQ.XLO BPU X BD !

Z 0 r 2ZxCK X AB -

Case 1. Now if the body ascends, and the gravity be as AB 2 + BD 3
BET being a circle, the line AC, which is proportional to the gravity

A13 2 i RT)2

will be -- T, -; and DP 2 or AP 2 + 2BAP + AB 2 + BD 2 will be

AK X Z + AC X Z or CK X Z ; and therefore the area DTV will be to
the area DPQ as DT 2 or I)B 2 to CK X Z.

Case 2. If the body ascends, and the gravity be as AB 2 —BD 2 , the
AT3 2 _ Til) 2

line AC will be-^-and DT 2 will be to DP 2 as DF 2 or DB 2

Z

to BP 2 —BD 2 or AP 2 + 2BAP + AB 2 —BD 2 , that is, to AK X Z +

AC X Z or CK X Z. And therefore the area DTV will be to the area
DPQ as DB 2 to CK X Z.

Case 3. And by the same reasoning, if the body descends, and therefore
the gravity is as BD 2 —AB 2 , and the line AC becomes equal to
TCD 2 _AB 2

----; the area DTV will be to the area DPQ as DB 2 to CK X

Z

Z: as above.

Since, therefore, these areas are always in this ratio, if for the area

Sec. 111.

OF NATURAL PHILOSOPHY.

2S5

DTY, by which the moment of the time, always equal to itself, is express¬
ed, there be put any determinate rectangle, as BD X ra, the area DPQ,,
that is, |BD X PQ, will be to BD X m as CK X Z to BI) 2 . And thence
PQ. X BD 3 becomes equal to2BD X m X CK X Z,and the moment KLON

of the area A6NK, found before, becomes

BP X BD X m
AB *

Prom the area

DET subduct its moment DTY or BD X ra, and there will remain
AP X BD X ni

--. Therefore the difference of the moments, that is, the

mo.nent of the difference of the areas, is equal to

AP X BD X rn
AB

and

therefore (because of the given quantity

BD X m

AB

-) as the velocity AP;

that is, as the moment of the space which the body describes in its ascent
or descent. And therefore the difference of the areas, and that space, in¬
creasing or decreasing by proportional moments, and beginning together or
vanishing together, are proportional. Q,.E.D.

Cor. If the length, which arises by applying the area DET to the line
BD, be called M ; and another length Y be taken in that ratio to the length
M, which the line DA has to the line DE; the space which a body, in a
resisting medium, describes in its whole ascent or descent, will be to the
space which a body, in a non-resisting medium, falling from rest, can de¬
scribe in the same time, as the difference of the aforesaid areas to

BD X Y 2

— -t-r— ) an( l therefore is given from the time given. For the space in a
Ad

non-resisting medium is in a duplicate ratio of the time, or as Y 2 ; and.

BD X Y 2

because BD and AB are given, as —jg-—. This area is equal to the

area

DA 2 X BD X M 5
DE 2 X AB

and the moment of M is m; and therefore the

, , . . DA 2 X BD X 2M X m

moment of this area is--" But this moment is to

Dht- X Ar5

the moment of the difference of the aforesaid areas DET and A6NK, viz., to

AP X Bl) X m DA’XBDxM, ,r>r> .. .r> OA 2 • . T^m
--, as-- to iBD X AP, or as into DET

to DAP; and, therefore, when the areas DET and DAP are least, in the

BD X Y 2

ratio of equality. Therefore the area —-and the difference of the

areas DET and A&NK, when all these areas are least, have equal moments;
and t re therefore equal. Therefore since the velocities, and therefore also
the sj aces in both mediums described together, in the beginning of the de¬
scent. or the end of the ascent, approach to equality, and therefore are then

286

THE MATHEMATICAL PRINCIPLES

[Book II

BD X V 2

one to another as the area ——, and the difference of the areas DET

and A6NK; and moreover since the space, in a non-resisting medium, is
BD X V 2

perpetually as-—, and the space, in a resisting medium, is perpetu¬

ally as the difference of the areas DET and A6NK ; it necessarily follows,
that the spaces, in both mediums, described in any equal times, are one to
BD X V 2

another as that area-- 4 —-, and the difference of the areas DET and

AB

AbNK. QJE.D.

SCHOLIUM.

The resistance of spherical bodies in fluids arises partly from the tena¬
city, partly from the attrition, and partly from the density of the medium.
And that part of the resistance which arises from the density of the fluid
is, as I said, in a duplicate ratio of the velocity; the other part, which
arises from the tenacity of the fluid, is uniform, or as the moment of the
time ; and, therefore, we might now proceed to the motion of bodies, which
are resisted partly by an uniform force, or in the ratio of the moments of
the time, and partly in the duplicate ratio of the velocity. But it is suf¬
ficient to have cleared the way to this speculation in Prop. VIII and IX
foregoing, and their Corollaries. For in those Propositions, instead of the
uniform resistance made to an ascending body arising from its gravity,
one may substitute the uniform resistance which arises from the tenacity
of the medium, when the body moves by its vis insita alone; and when the
body ascends in a right line, add this uniform resistance to the force of
gravity, and subduct it when the body descends in a right line. One
might also go on to the motion of bodies which are resisted in part uni¬
formly, in part in the ratio of the velocity, and in part in the duplicate
ratio of the same velocity. And I have opened a way to this in Prop.
XIII and XIY foregoing, in which the uniform resistance arising from the
tenacity of the medium may be substituted for the force of gravity, or be
compounded with it as before. But I hasten to other things.

Sec. IV'.]

OF NATURAL PHILOSOPHY.

287

SECTION IV.

Of the circular motion of bodies in resisting mediums.

LEMMA III.

Let PQR be a spiral cutting all the radii SP, SQ, SR, $*c., in equal
angles. Draw the right line PT touching the spiral in any point P,
and cutting the radius SQ in T; draw PO, QO perpendicular to
the spiral , and meeting in O, and join SO. J say, that if the points
P and Q approach and coincide, the angle PSO will become a right
angle , and the ultimate ratio of the rectangle TQ X 2PS to Pol 2 will
be the ratio of equality.

For from the right angles OPQ, OQR, sub¬
duct the equal angles SPQ, SQR, and there
will remain the equal angles OPS, OQS.

Therefore a circle which passes through the
points OSP will pass also through the point
Q. Let the points P and Q coincide, and
this circle will touch the spiral in the place
of coincidence PQ, and will therefore cut the
right line OP perpendicularly. Therefore OP will become a diameter of
this circle, and the angle OSP, being in a semi-circle, becomes a right
one. Q.E.D.

Draw QD, SE perpendicular to OP, and the ultimate ratios of the lines
will be as follows : TQ to PD as TS or PS to PE, or 2PO to 2PS • and
PD to PQ as PQ to 2PO; and, ex cequo perturbate, to TQ to PQ as PQ
to 2PS. Whence PQ 2 becomes equal to TQ X 2PS. Q.E.D.

PROPOSITION XV. THEOREM XII.

Tf the density of a medium in each place thereof be reciprocal 1 y as the
distance of the places from an immovable centre, and the centripetal
force be in the duplicate ratio of the density ; I say, that a body may
revolve in a spiral which cuts all the radii drawn from that centre
in a given angle.

Suppose every thing to be as in the forego¬
ing Lemma, and produce SQ to V so that SV
may be equal to SP. In any time let a body,
in a resisting medium, describe the least arc
PQ, and in double the time the least arc PR ;
and the decrements of those arcs arising from
the resistance, or their differences from the
arcs which would be described in a non-resist¬
ing medium in the same times, will be to each
other as the squares of the times in which they
are generated; therefore the decrement of the

288

THE MATHEMATICAL PRINCIPLES

[Book 11

arc PQ is the fourth part of the decrement of the arc PR. Whence also
if the area QSr be taken equal to the area PSQ, the decrement of the arc
PQ will be equal to half the lineola Rr and therefore the force of resist¬
ance and the centripetal force are to each other as the lineola iRr and TQ
which they generate in the same time. Because the centripetal force with
which the body is urged in P is reciprocally as SP 2 , and (by Lem. X,
Book I) the lineola TQ, which is generated by that force, is in a ratio
compounded of the ratio of this force and the duplicate ratio of the time
in which the arc PQ is described (for in this case I neglect the resistance,
as being infinitely less than the centripetal force), it follows that TQ X
SP 2 , that is (by the last Lemma), |PQ 2 X SP, will be in a duplicate ra¬
tio of the time, and therefore the time is as PQ X -s/SP ; and the velo¬
city of the body, with which the arc PQ is described in that time, as
PQ 1

PQ X \/SP ° r 1/SP’ ^ at ^duplicate ra ^° of SP reciprocally.

And, by a like reasoning, the velocity with whioh the arc QRis described,
is in the subduplicate ratio of SQ reciprocally. Now those arcs PQ and
QR are as the describing velocities to each other; that is, in the subdu¬
plicate ratio of SQ to SP, or as SQ to v/SP X SQ; and, because of the
equal angles SPQ, SQ?', and the equal areas PSQ, QSr, the arc PQ is to
the arc Qr as SQ to SP. Take the differences of the proportional conse¬
quents, and the arc PQ will be to the arc Rr as SQ to SP

or ^VQ. For the points P and Q coinciding

^SP X SQ,
r, the ultimate ratio of SP —
v'SP X SQ to |VQ is the ratio of equality. Because the decrement of
the arc PQ arising from the resistance, or its double Rr, is as the resistance

Rr

and the square of the time conjunctly, the resistance will be as p Q 0 ^ ^ p.

Rr

But PQ was to Rr as SQ to fVQ, and thence becomes as

PQ 2 X SP

iVQ

r OS

or ns

^~p 7 . For the points P and Q coinciding,

PQ X SP X SQ' OP X
SP and SQ coincide also, and the angle PVQ becomes a right one; and,
because of the similar triangles PVQ, PSO, PQ. becomes to |VQ as OP
OS

to -jOS. Therefore -~r.j —is as the resistance, that is, in the ratio of

the density of the medium in P and the duplicate ratio of the velocity

conjunctly. Subduct the duplicate ratio of the velocity, namely, the ratio

1 OS

gp, and there will remain the density of the medium in P. as Q p gp

Let the spiral be given, and, because of the given ratio of OS to OP, the

density of the medium in P will be as~p. Therefore in a medium whose

OF NATURAL PHILOSOPHY.

2S9

Sec. IV.]

density is reciprocally as SP the distance from the centre, a body will re¬
volve in this spiral. Q.E.D.

Cor. 1. The velocity in any place P, is always the same wherewith a
body in a non-resisting medium with the same centripetal force would re¬
volve in a circle, at the same distance SP from the centre.

Cor. 2. The density of the medium, if the distance SP be given, is as

OS

OP’

but if that distance is not given, as

OS

OP X SP*

And thence a spiral

may be fitted to any density of the medium.

Cor. 3. The force of the resistance in any place P is to the centripetal
force in the same place as AOS to OP. For those forces are to each other
AVO X PQ. , APQ 2

as ARr and TO, or as —

so

and

*gp—-, that is, as a VO and PO,

or AOS and OP. The spiral therefore being given, there is given the pro¬
portion of the resistance to the centripetal force ; and. vice versa , from that
proportion given the spiral is given.

Cor. 4. Therefore the body cannot revolve in this spiral, except where
the force of resistance is less than half the centripetal force. Let the re¬
sistance be made equal to half the centripetal force, and the spiral will co¬
incide with the right line PS, and in that right line the body will descend
to the centre with a velocity that is to the velocity, with which it was
proved before, in the case of the parabola (Theor. X, Book I), the descent
would be made in a non-resisting medium, in the subduplicate ratio of
unity to the number two. And the times of the descent will be here recip¬
rocally as the velocities, and therefore given.

Cor. 5. And because at equal distances
from the centre the velocity is the same in the
spiral PQ,R as it is in the right line SP, and
the length of the spiral is to the length of the
right line PS in a given ratio, namely, in the
ratio of OP to OS; the time of the descent in
the spiral will be to the time of the descent in
the right line SP in the same given ratio, and
therefore given.

Cor. 6 . If from the centre S, with any two
given intervals, two circles are described; and
these circles remaining, the angle which the spiral makes with the radius
PS be any how changed; the number of revolutions which the body can
complete in the space between the circumferences of those circles, going

PS

round in the spiral from one circumference to another, will be as or as
• Uo

the tangent of the angle which the spiral makes with the radius PS ; and

19

290

THE MATHEMATICAL PRINCIPLES

[Book II

the time of the same revolutions will be as ^g, that is, as the secant of the

3ame angle, or reciprocally as the density of the medium.

Cor. 7. If a body, in a medium whose density is reciprocally as the dis¬
tances of places from the centre, revolves in any curve AEB about that
centre, and cuts the first radius AS in the same
angle in B as it did before in A, and that with a
velocity that shall be to its first velocity in A re¬
ciprocally in a subduplicate ratio of the distances
from the centre (that is, as AS to a mean propor¬
tional between AS and BS) that body will con¬
tinue to describe innumerable similar revolutions
BFC, CGD, &c., and by its intersections will
distinguish the radius AS into parts AS, BS, CS, DS, (fee., that are con¬
tinually proportional. But the times of the revolutions will be as the
perimeters of the orbits AEB, BFC, CGD, (fee., directly, and the velocities

.2 -2

at the beginnings A, B, C of those orbits inversely ; that is as AS 2 . BS 2 ,

CS 2 . And the whole time in which the body will arrive at the centre,
will be to the time of the first revolution as the sum of all the continued

3 3. 3.

proportionals AS 2 , BS 2 , CS 2 , going on ad infinitum, to the first term

Ji 3 3

AS 2 ; that is, as the first term AS 2 to the difference of the two first AS 2

3

— BS 2 , or as f AS to AB very nearly. Whence the whole time may be
easily found.

Cor. 8 . From hence also may be deduced, near enough, the motions of
bodies in mediums whose density is either uniform, or observes any other
assigned law. From the centre S, with intervals SA, SB, SC, (fee., con¬
tinually proportional, describe as many circles; and suppose the time of
the revolutions between the perimeters of any two of those circles, in the
medium whereof we treated, to be to the time of the revolutions between
the same in the medium proposed as the mean density of the proposed me¬
dium between those circles to the mean density of the medium whereof wc
treated, between the same circles, nearly : and that the secant of the angle
in which the spiral above determined, in the medium whereof we treated,
cuts the radius AS, is in the same ratio to the secant of the angle in which
the new spiral, in the proposed medium, cuts the same radius: and also
that the number of all the revolutions between the same two circles is nearly
as the tangents of those angles. If this be done every where between e very
two circles, the motion will be continued through all the circles. And by
this means one may without difficulty conceive at what rate and in what
time bodies ought to revolve in any regular medium.

•Sec. IY.l

OF NATURAL PHILOSOPHY.

291

Cor. 9. And although these motions becoming eccentrical should be
performed in spirals approaching to an oval figure, yet, conceiving the
several revolutions of those spirals to be at the same distances from each
other, and to approach to the centre by the same degrees as the spiral above
described, we may also understand how the motions of bodies may be per¬
formed in spirals of that kind.

PROPOSITION XYI. THEOREM XIII.

If the density of the medium in each of the places be reciprocally as the
distance of the places from the immoveable centre, and the centripetal
force be reciprocally as any power of the same distance, I say, that the
body may revolve in a spiral intersecting all the radii drawn from
that centre in a given angle .

This is demonstrated in the same manner as
the foregoing Proposition. For if the centri¬
petal force in P be reciprocally as any power
SP n -f 1 of the distance SP whose index is n
+ 1; it will be collected, as above, that the
time in which the body describes any arc PQ,

will be as PQ, X PS 2Q ; and the resistance in
n ’ Rr T— X YQ

P 38 PQ. 3 X SP"’ 01 as PQ x SP" X SQ’ and

, 1 — X OS . 1 — in X OS .

therefore as q)T~ x gp ' , 7^TT» t “ at 1S > (because-gp-is a given

quantity), reciprocally as SP n -f 1 . And therefore, since the velocity is recip-

JL

rocally as SP 2n , the density in P will be reciprocally as SP.

Cor. 1. The resistance is to the centripetal force as 1 — ±n X OS
to OP.

Cor. 2. If the centripetal force be reciprocally as SP 3 , 1 — \n will be
=== 0; and therefore the resistance and density of the medium will be
nothing, as in Prop. IX, Book I.

Cor. 3. If the centripetal force be reciprocally as any power of the ra¬
dius SP, whose index is greater than the number 3, the affirmative resist¬
ance will be changed into a negative.

SCHOLIUM.

This Proposition and the former, which relate to mediums of unequal
density, are to be understood of the motion of bodies that are so small, that
the greater density of the medium on one side of the body above that on
the other is not to be considered. I suppose also the resistance, cceteris
paribus, to be proportional to. its density. Whence, in mediums whose

292

THE MATHEMATICAL PRINCIPLES

IBook II

force of resistance is not as the density, the density must be so much aug¬
mented or diminished, that either the excess of the resistance may be taken
away, or the defect supplied.

PROPOSITION XVII. PROBLEM IV

To find the centripetal for ce and the resisting force of the medium, by
which a body, the law of the velocity being given, shall revolve in a
given spiral.

Let that spiral be PQR. From the velocity,
with which the body goes over the very small arc
PQ,, the time will be given: and from the altitude
TO, which is as the centripetal force, and the
square of the time, that force will be given. Then
from the difference RS?* of the areas PSQ, and
Q,SR described in equal particles of time, the re¬
tardation of the body will be given; and from
the retardation will be found the resisting force
and density of the medium.

PROPOSITION XVIII. PROBLEM V.

The law of centripetal force being given, to find the density of the me¬
dium in each of the places thereof, by which ' a body may describe a
given spiral.

From the centripetal force the velocity in each place must be found;
then from the retardation of the velocity the density of the medium is
found, as in the foregoing Proposition.

But I have explained the method of managing these Problems in the
tenth Proposition and second Lemma of this Book; and will no longer
detain the reader in these perplexed disquisitions. I shall now add some
things relating to the forces of progressive bodies, and to the density and
resistance of those mediums in which the motions hitherto treated of, and
those akin to them, are performed.

Sec. V.]

OF NATURAL PHILOSOPHY.

293

SECTION V.

l>f the density and compression of fluids ; and of hydrostatics.

THE DEFINITION OF A FLUID.

A fluid is any body whose parts yield to any force impressed on it,
by yielding , are easily moved among themselves.

PROPOSITION XIX. THEOREM XIV
All the parts of a homogeneous and unmoved fluid included in any nn~
moved vessel , and compressed on every side (setting aside the consider -
ation of condensation } gravity , and all centripetal forces ), will be
equally pressed on every side , and remain, in their places without any
motion arising from that pressure.

Case 1. Let a fluid be included in the spherical A

vessel ABC, and uniformly compressed on every
side: 1 say, that no part of it will be moved by
that pressure. For if any part, as O, be moved,
all such parts at the same distance from the centre
on every side must necessarily be moved at the
same time by a like motion ; because the pressure
of them all is similar and equal; and all other B
motion is excluded that does not arise from that
pressure. But if these parts come all of them nearer to the centre, the
fluid must be condensed towards the centre, contrary to the supposition.
If they recede from it, the fluid must be condensed towards the circumfer¬
ence ; which is also contrary to the supposition. Neither can they move
in any one direction retaining their distance from the centre, because for
the same reason, they may move in a contrary direction ; but the sami
part cannot be moved contrary ways at the same, time. Therefore no
part of the fluid will be moved from its place. Q,.E.D.

Case 2. I say now, that all the spherical parts of this fluid are equally
pressed on every side. For let EF be a spherical part of the fluid ; if this
be not pressed equally on every side, augment the lesser pressure till it be
pressed equally on every side; and its parts (by Case 1) will remain in
their places. But before the increase of the pressure, they would remain
in their places (by Case 1); and by the addition of a new pressure they
will be moved, by the definition of a fluid, from those places. Now these
two conclusions contradict each other. Therefore it was false to say that
the sphere EF was not pressed equally on every side. Q.E.D.

Case 3. I say besides, that different spherical parts have equal pressures.
For the contiguous spherical parts press each other mutually and equally
in the point of contact (by Law III). But (by Case 2) they are pressed on
every side with the same force. Therefore any two spherical parts *iot

1 HE MATHEMATICAL PRINCIPLES

291

[Book II.

contiguous, since an intermediate spherical part can touch both, will be
pressed with the same force. Q.E.D.

Case 4. I say now, that all the parts of the fluid are every where press¬
ed equally. For any two parts may be touched by spherical parts in any
points whatever; and there they will equally .press those spherical parts
(by Case 3). and are reciprocally equally pressed by them (by Law III).
Q.E.D.

Case 5. Since, therefore, any part GHI of the fluid is inclosed by the
rest of the fluid as in a vessel, and is equally pressed on every side ; and
also its parts equally press one another, and are at rest among themselves;
it is manifest that all the parts of any fluid as GHI, which is pressed
equally on every side, do press each other mutually and equally, and are at
rest among themselves. Q.E.D.

Case 6. Therefore if that fluid be included in a vessel of a yielding
substance, or that is not rigid, and be not equally pressed on every side,
the same will give way to a stronger pressure, by the Definition of fluidity.

Case 7. And therefore, in an inflexible or rigid vessel, a fluid will not
sustain a stronger pressure on one side than on the other, but will give
way to it, and that in a moment of time; because the rigid side of the
vessel does not follow the yielding liquor. But the fluid, by thus yielding,
will press against the opposite side, and so the pressure will tend on every
side to equality. And because the fluid, as soon as it endeavours to recede
from the part that is most pressed, is withstood by the resistance of the
vessel on the opposite side, the pressure will on every side be reduced to
equality, in a moment of time, without any local motion : and from thence
the parts of the fluid (by Case 5) will press each other mutually and equal¬
ly, and be at rest among themselves. Q.E.D.

Cor. Whence neither will a motion of the parts of the fluid among
themselves be changed by a pressure communicated to the external super¬
ficies, except so far as either the figure of the superficies may be somewhere
altered, or that all the parts of the fluid, by pressing one another more in¬
tensely or remissly, may slide with more or less difficulty among them-
Belves.

PROPOSITION XX. THEOREM XV.

If all the parts of a spherical fluid, homogeneous at equal distances from
the centre, lying on a spherical concentric bottom\, gravitate towards
the centre of the ichole, the bottom will sustain the weight of a cylin¬
der, whose base is equal to the superficies of the bottom, and whose al¬
titude is the same with that of the incumbent fluid.

Let I)HM be the superficies of the bottom, and AEI the upper super¬
ficies of the fluid. Let the fluid be distinguished into concentric orbs of
squal thickness, by the innumerable spherical superficies *3FK, CGL : and

Sec. V

OF NATURAL PHILOSOPHY.

295

conceive the force of gravity to act only in the
upper superficies of every orb, and the actions
to be equal on the equal parts of all the su¬
perficies. Therefore the upper superficies AE
is pressed by the single force of its own grav¬
ity, by which all the parts of the upper orb,
and the second superficies BFK, will (by
Prop. XIX), according to its measure, be
equally pressed. The second superficies BFK
is pressed likewise by the force of its own
gravity, which, added to the former force,
makes the pressure double. The third superficies CGL is, according to its
measure, acted on by this pressure and the force of its own gravity besides,
which makes its pressure triple. And in like manner the fourth superfi¬
cies receives a quadruple pressure, the fifth superficies a quintuple, and so
on. Therefore the pressure acting on every superficies is not as the solid
quantity of the incumbent fluid, but as the number of the orbs reaching
to the upper surface of the fluid ; and is equal to the gravity of the low'est
orb multiplied by the number of orbs: that is, to the gravity of a solid
whose ultimate ratio to the cylinder above-mentioned (when the number of
the orbs is increased and their thickness diminished, ad infinitum ,, so that
the action of gravity from the lowest superficies to the uppermost may be¬
come continued) is the ratio of equality. Therefore the lowest superficies
sustains the weight of the cylinder above determined. Q,.E.D. And by a
like reasoning the Proposition will be evident, where the gravity of the
fluid decreases in any assigned ratio of the distance from the centre, and
also where the fluid is more rare above and denser below. Q.E.D.

Cor. 1. Therefore the bottom is not pressed by the whole weight of the
incumbent fluid, but only sustains that part of it which is described in the
Proposition ; the rest of the weight being sustained archwise by the spheri¬
cal figure of the fluid.

Cor. 2. The quantity of the pressure is ^the same always at equal dis¬
tances from the centre, whether the superficies pressed be parallel to the
horizon, or perpendicular, or oblique; or whether the fluid, continued up¬
wards from the compressed superficies, rises perpendicularly in a rectilinear
direction, or creeps obliquely through crooked cavities and canals, whether
those passages be regular or irregular, wide or narrow. That the pressure
is not altered by any of these circumstances, may he collected by applying
the demonstration of this Theorem to the several cases of fluids.

Cor. 3. From the same demonstration it may also be collected (by Prop.
XIX), that the parts of a heavy fluid acquire no motion among themselves
by the pressure of the incumbent weight, except that motion which arises
from condensation.

296

THE MATHEMATICAL PRINCIPLES

[Bcok II

Cor. 4. And therefore if another body of the same specific gravity, in¬
capable of condensation, be immersed in this fluid, it will acquire no mo¬
tion by the pressure of the incumbent weight: it will neither descend nor .
ascend, nor change its figure. If it be spherical, it will remain so, notwith¬
standing the pressure; if it be square, it will remain square; and that,
whether it be soft or fluid; whether it swims freely in the fluid, or lies at
the bottom. For any internal part of a fluid is in the same state with the
submersed body ; and the case of all submersed bodies that have the same
magnitude, figure, and specific gravity, is alike. If a submersed body, re¬
taining its weight, should dissolve and put on the form of a fluid, this
body, if before it would have ascended, descended, or from any pressure as¬
sume a new figure, would now likewise ascend, descend, or put on a new
figure; and that, because its gravity and the other causes of its motion
remain. But (by Case 5, Prop. XtX; it would now be at rest, and retain
its figure. Therefore also in the former case.

Cor. 5. Therefore a body that is specifically heavier than a fluid con¬
tiguous to it will sink; and that which is specifically lighter will ascend,
and attain so much motion and change of figure as that excess or defect of
gravity is able to produce. For that excess or defect is the same thing as an
impulse, by which a body, otherwise in equilibria with the parts of the
fluid, is acted on; and may be compared with the excess or defect of a
weight in one of the scales of a balance.

Cor. 6. Therefore bodies placed in fluids have a twofold gravity * the
one true and absolute, the other apparent, vulgar, and comparative. Ab¬
solute gravity is the whole force with which the body tends downwards;
relative and vulgar gravity is the excess of gravity with which the body
tends downwards more than the ambient fluid. By the first kind of grav¬
ity the parts of all fluids and bodies gravitate in their proper places; and
therefore their weights taken together compose the weight of the whole.
For the whole taken together is heavy, as may be experienced in vessels
full of liquor ; and the weight of the whole is equal to the weights of all
the parts, and is therefore composed of them. By the other kind of grav¬
ity bodies do not gravitate in their places; that is, compared with one
another, they do not preponderate, but, hindering one another’s endeavours
to descend, remain in their proper places, as if they were not heavy. Those
things which are in the air, and do not preponderate, are commonly looked
on as not heavy. Those which do preponderate are commonly reckoned
heavy, in as much as they are not sustained by the weight of the air. The
common weights are nothing else but the excess of the true weights above
the weight of the air. Hence also, vulgarly, those things are called light
which are less heavy, and, by yielding to the preponderating air, mount
upwards. But these are only comparatively lig anu not truly so, because

hey descend in vacuo. Thus, in water, bodies by their greater or

OF NATURAL PHILOSOPHY.

29?

Sec. V.]

less gravity, descend or ascend, are comparatively and apparently heavy or
light; and their comparative and apparent gravity or levity is the excess
.or defect by which their true gravity either exceeds the gravity of the
water or is exceeded by it. But those things which neither by preponder¬
ating descend, nor, by yielding to the preponderating fluid, ascend, although
by their true weight they do increase the weight of the whole, yet com¬
paratively, and in the sense of the vulgar, they do not gravitate in the wa¬
ter. For these cases are alike demonstrated.

Cor. 7. These things which have been demonstrated concerning gravity
take place in any other centripetal forces.

Cor. 8. Therefore if the medium in which any body moves be acted on
either by its own gravity, or by any other centripetal force, and the body
be urged more powerfully by the same force ; the difference of the forces is
that very motive force, which, in the foregoing Propositions, I have con¬
sidered as a centripetal force. But if the body be more lightly urged by
that force, the difference of the forces becomes a centrifugal force, and is tc
be considered as such.

Cor. 9. But since fluids by pressing the included bodies do not
change their external figures, it appears also (by Cor. Prop. XIX) that they
will not change the situation of their internal parts in relation to onf
another ; and therefore if animals were immersed therein, and that all sen¬
sation did arise from the motion of their parts, the fluid will neither hurt
the immersed bodies, nor excite any sensation, unless so far as those bodies
may be condensed by the compression. And the case is the same of any
system of bodies encompassed with a compressing fluid. All the parts of
the system will be agitated with the same motions as if they were placed
in a vacuum, and would only retain their comparative gravity ; unless so
far as the fluid may somewhat resist their motions, or be requisite to con-
glutinate them by compression.

PROPOSITION XXI. THEOREM XYI.

Let the density of any fluid be proportional to the compression , and its
parts be attracted downwards by a centripetal force reciprocally pro¬
portional to the distances from the centre: I say, that , if those dis¬
tances be taken continually proportional , the detisities of the fluid at
the same distances will be also continually proportional .

Let ATV denote the spherical bottom of the fluid, S the centre, S A, SB,
SC, SD, SE, SF, &c., distances continually proportional. Erect the per¬
pendiculars AH, BI, CK, DL, EM, FN, (fee., which shall be as the densi¬
ties of the medium in the places A, B, C, D, E, F; and the specific grav-

AH BI CK ' ....

A§’ BS’ "CS’ or * walca 1S ail one? a& '

ities in those places will be as

29S

THE MATHEMATICAL PRINCIPLES

[Book II.

G

-tO

iN

X

E

D

xxr

C

B

AH BI CK

AiT'* BC’ CD’ ^ C ‘ Suppose, these gravities to be uniformly continued

from A to B, from B to C, from C to D, (fee., the decrements in the points
B, C, D, (fee., being taken by steps. And these gravi¬
ties drawn into the altitudes AB, BC, CD, (fee., will
give the pressures AH, BI, CK, (fee., by which the bot¬
tom ATY is acted on (by Theor. XY). Therefore the
particle A sustains all the pressures AH, BI, CK, DL,
(fee., proceeding in infinitum; and the particle B sus¬
tains the pressures of all but the first AH; and the par¬
ticle C all but the two first AH, BI; and so on : and
therefore the density AH of the first particle A is to
' v the density BI of the second particle B as the sum of
all AH -+■ BI + CK 4- DL, in infinitum , to the sum of
all BI 4* CK 4- DL, (fee. And BI the density of the second particle B is
to CK the density of the third C, as the sum of all BI 4- CK + DL, (fee.,
to the sum of all CK 4- DL, (fee. Therefore these sums are proportional
to their dilferences AH, BI, CK, (fee., and therefore continually propor¬
tional (by Lem. 1 of this Book); and therefore the differences AH, BI,
CK, (fee., proportional to the sums, are also continually proportional.
Wherefore since the densities in the places A, B, C, (fee., are as AH, BI,
CK, (fee., they will also be continually proportional. Proceed intermis-
sively, and, ex ccquo, at the distances SA, SC, SE, continually proportional,
the densities AH, CK, EM will be continually proportional. And by the
same reasoning, at any distances SA, SD, SG, continually proportional,
the densities AH, I)L, GO, will be continually proportional. Let now the
points A, B, C, D, E, (fee., coincide, so that the progression of the specif.c
gravities from the bottom A to the top of the fluid may be made continual;
and at any distances SA, SD, SG, continually proportional, the densities
AH, DL, GO, being all along continually proportional, will still remain
continually proportional. Q.E.D.

Cor. Hence if the density of the fluid in two places,
as A and E, be given, its density in any other place Q
may be collected. With the centre S, and the rectan¬
gular asymptotes SQ, SX, describe an hyperbola cut¬
ting the perpendiculars AH, EM, QT in a, e, and 9 ,
as also the perpendiculars HX, MY, TZ, let fall upon
the asypmtote SX, in //, m, and t. Make the area
YrniZ to the given area Y mhX. as the given area
EeqQ to the given area EmA; and the line Z t produced will cut off the
line QT proportional to the density. For if the lines SA, SE, SQ are
continually proportional, the areas Ee 9 Q, EJeaA will be equal, and thence

.Sec. V.J

OF NATURAL PHILOSOPHY.

299

the areas Yml7i, XAwY, proportional to them, will be also equal; and
the lines SX, SY, SZ, that is, AH, EM, Q,T continually proportional, as
they ought to be. And if the lines SA, SE, SQ, obtain any other order
in the series of continued proportionals, the lines AH, EM, Q,T, because
of the proportional hyperbolic areas, will obtain the same order in another
series of quantities continually proportional.

PROPOSITION XXII THEOREM XVII.

Let the density of any fluid be proport ional to the compression , and its
parts be attracted downwards by a gravitation reciprocally propor¬
tional to the squares of the distances from the centre : I say , that if
the distances be taken in harmonic progression , the densities of the
fluid at those distances will be in a geometrical progression.

Let S denote the centre, and SA,

SB, SC, SD, SE, the distances in . j _ iiv

geometrical progression. Erect the ^
perpendiculars AH, BI, CK, (fee., ^
which shall be as the densities of
the fluid in the places A, B, C, D, B
E, (fee., and the specific gravities
thereof in those places will be as
AH BI CK t

SU’ SB 1 ’ SC*’ &c ' Su PP osethese . "

gravities to be uniformly continued, the first from A to B, the second from
B to C, the third from C to I), &c. And these drawn into the altitudes
AB, BC, CD, DE, (fee., or, which is the same thing, into the distances SA,

, . , . AH BI CK

SB, SC, (fee., proportional to those altitudes, will give -g-^, gg, -gg, (fee..

the exponents of the pressures. Therefore since the densities are as th*
sums of those pressures, the differences AH — BI, BI—CK, (fee., of tb.*

HbbmSh

densities will be as the differences of those sums

AH BI CK

, (fee. With

IT 1 u CIV VIAAV U1UV1 VUVVO V .L V11VOV OU111W 1

the centre S, and the asymptotes SA, S#, describe any hyperbola, cutting
the perpendiculars AH, BI, CK, (fee., in a, b , c, (fee., and the perpendicu¬
lars H£, lu, Kw, let fall upon the asymptote Sx, in h, i, k ; and the dif¬
ferences of the densities tu , uw, (fee., will be as (fee. And the

oA oo

. , . o o AH X th BI X ui .

rectangles tu X th , uw X ut, (fee., or tp, uq , (fee., as —g—— > -gg—, (fee.,

that is, as Aa, B6, (fee. For, by the nature of the hyperbola, SA is to AH
\ AH X th

or St as th to A o, and therefore —— ; is equal to Aa. And, by a like

300

THE MATHEMATICAL PRINC. PLES

[Book II.

reasoning, —^— is equal to Bb, (fee. But A a, B6, Cc, tfec., are continu¬
ally proportional, and therefore proportional to their differences A a — B b,
B6 — C c, <fcc., therefore the rectangles tp, uq, &c., are proportional to those
differences; as also the sums of the rectangles tp + uq, or tp + u,q + wr
to the sums of the differences A a — C c or A a — T)d. Suppose several of
these terms, and the sum of all the differences, as A a — F f, will be pro¬
portional to the sum of all the rectangles, as zthn. Increase the number
of terms, and diminish the distances of the points A, B, C, (fee., in infini¬
tum, and those rectangles will become equal to the hyperbolic area zthn,
and therefore the difference A a — Ff i* proportional to this area. Take
now any distances, as SA, SD, SF, in harmonic progression, and the dif¬
ferences A a — D d, Dt/ — Ff will be equal; and therefore the areas thlx,
xlnz, proportional to those differences will be equal among themselves, and
the densities St, Sx, Sz, that is, AH, DL, FN, continually proportional.
Q.E.D.

Cor. Hence if any two densities of the fluid, as AH and BI, be given,
the area thin, answering to their difference tu, will be given; and thence
the density FN will be found at any height SF, by taking the area thnz to
that given area thin as the difference A a — Ff to the difference Aa — B/>.

SCHOLIUM.

By a like reasoning it may be proved, that if the gravity of the particles
of a fluid be diminished in a triplicate ratio of the distances from the centre;
and the reciprocals of the squares of the distances SA, SB, SC, (fee., (namely,

SA 3 SA 3 SA 3
SA7 j SB 3 ’ SC 2

) be taken in an arithmetical progression, the densities AH,

BI, CK, (fee., will be in a geometrical progression. And if the gravity be
diminished in a quadruplicate ratio of the distances, and the reciprocals of

the cubes of the distances (as

SA 4 SA 4 SA 4

SA 3, SB 3, SC

rjT, (fee.,) be taken in arithmeti¬

cal progression, the densities AH, BI, CK, (fee., will be in geometrical pro¬
gression. And so in infinitum. Again; if the gravity of the particles of
the fluid be the same at all distances, and the distances be in arithmetical
progression, the densities will be in a geometrical progression as Dr. Hal¬
ley has found. If the gravity be as the distance, and the squares of the
distances be in arithmetical progression, the densities will be in geometri¬
cal progression. And so in infinitum. These things will be so, when the
density of the fluid condensed by compression is as the force of compres¬
sion ; or, which is the same thing, when the space possessed by the fluid is
reciprocally as this force. Other laws of condensation may be supposed,
as that the cube of the compressing force may be as the biquadrate of the

OF NATURAL PHILOSOPHY.

301

Sec. V.]

de isity ; or the triphcate ratio of the force the same with the quadruplicate
ratio of the density : in which case, if the gravity be reciprocally as the
square of the distance from the centre, the density will be reciprocally as
the cube of the distance. Suppose that the cube of the compressing force
be as the quadrato-cube of the density; and if the gravity be reciprocally
as the square of the distance, the density will be reciprocally in a sesqui-
plicate ratio of the distance. Suppose the compressing force to be in a du¬
plicate ratio of the density, and the gravity reciprocally in a duplicate ra¬
tio of the distance, and the density will be reciprocally as*the distance.
To run over all the cases that might be offered would be tedious. But as
to our own air, this is certain from experiment, that its density is either
accurately, or very nearly at least, as the compressing force; and therefore
the density of the air in the atmosphere of the earth is as the weight of
the whole incumbent air, that is, as the height of the mercury in the ba¬
rometer.

PROPOSITION XXIII. THEOREM XVIII.

If a fluid be composed of particles mutually flying each other , and the
density be as the compression , the centrifugal forces of the particles
will be reciprocally proportional to the distances of their centres. And ,
vice versa, particles flying each other, with forces that are reciprocally
proport ional to the distances of their centres , compose an elastic fluid ,
lohose density is as the compression.

which that square db urges the inclosed fluid as the densities of the me¬
diums are to each other, that is, as ab 3 to AB 3 . But the pressure with
which the square DB urges the included fluid is to the pressure with which
the square DP urges the same fluid as the square DB to the square DP,
that is, as AB 2 to ab 2 . Therefore, ex cequo , the pressure witli which the
square DB urges the fluid is to the pressure with which the square db
urges the fluid as ab to AB. Let the planes FGH,fgh, be drawn through
the middles of the two cubes, and divide the fluid into two parts. These
parts will press each other mutually with the same forces with which they

302

THE MATHEMATICAL PRINCIPLES

TBook II.

are themselves pressed by the planes AC, ac, that is, in the proportion of
ab to AB : and therefore the centrifugal forces by which these pressures
are sustained are in the same ratio. The number of the particles being
equal, and the situation alike, in both cubes, the forces which all the par¬
ticles e&ert, according to the planes FGH,/g7i, upon all, are as the forces
which each exerts on each. Therefore the forces which each exerts on
each, according to the plane FCH in the greater cube, are to the forces
which each exerts on each, according to the plane fgh in the lesser cube,
as ab to AB/that is, reciprocally as the distances of the particles from each
other. Q.E.D.

And, vice versa , if the forces of the single particles are reciprocally as
the distances, that is, reciprocally as the sides of the cubes AB, ab ; the
sums of the forces will be in the same ratio, and the pressures of the sides
DB. db as the sums of the forces; and the pressure of the square DP to
the pressure of the side DB as ab 2 to AB 2 . And, ex ceqvo , the pressure of
the square DP to the pressure of the side db as ab 3 to AB 3 ; that is, the
force of compression in the one to the force of compression in the other as
the density in the former to the density in the latter. Q.E.D.

SCHOLIUM.

By a like reasoning, if the centrifugal forces of the particles are recip¬
rocally in the duplicate ratio of the distances between the centres, the cubes
of the compressing forces will be as the biquadrates of the densities. If
the centrifugal forces be reciprocally in the triplicate or quadruplicate ratio
cf the distances, the cubes of the compressing forces wdllbeas thequadrato-
cubes, or cubo-cubes of the densities. And universally, if D be put for the
distance, and E for the density of the compressed fluid, and the centrifugal
forces be reciprocally as any power D n of the distance, whose index is the
number n, the compressing forces will be as the cube roots of the power
E n + 2 , whose index is the number n + 2 ; and the contrary. All these
things are to be understood of particles whose centrifugal forces terminate
in those particles that are next them, or are diffused not much further.
We have an example of this in magnetical bodies. Their attractive vir¬
tue is terminated nearly in bodies of their own kind that are next them.
The virtue of the magnet is contracted by the interposition of an iron
plate, and is almost terminated at it: for bodies further off are not attracted
by the magnet so much as by the iron plate. If in this manner particles repel
others of their own kind that lie next them, but do not exert their virtue
on the more remote, particles of this kind will compose such fluids as are
treated of in this Proposition. If the virtue of any particle diffuse itself
every way in infinitum , there will be required a greater force to produce
an equal condensation of a greater quantity of the fluil. But whether

Sec. VI.]

OF NATURAL PHILOSOPHY.

303

elastic fluids do really consist of particles so repelling each other, is a phy-
sical question. We have here demonstrated mathematically the property
of fluids consisting of particles of this kind, that hence philosophers may
take occasion to discuss that question.

SECTION VI.

Of the motion and resistance of fnnependulnus bodies.

PROPOSITION XXIV. THEOREM XIX.

The quantities of matter in, f unependulous bodies , whose centres of oscil¬
lation, are equally distant from the centre of suspension , are in a, ratio
compounded of the ratio of the weights and the duplicate ratio of the
times of the oscillations in vacuo.

For the velocity which a given force can generate in a given matter in
a given time is as the force and the time directly, and the matter inversely.
The greater the force or the time is, or the less the matter, the greater ve¬
locity will be generated. This is manifest from the second Law of Mo¬
tion. Now if pendulums are of the same length, the motive forces in places
equally distant from the perpendicular are as the weights : and therefore
if two bodies by oscillating describe equal arcs, and those arcs are divided
into equal parts; since the times in which the bodies describe each of the
correspondent parts of the arcs are as the times of the whole oscillations,
the velocities in the correspondent parts of the oscillations will be to each
other as the motive forces and the whole times of the oscillations directly,
and the quantities of matter reciprocally : and therefore the quantities of
matter are as the forces and the times of the oscillations directly and the
velocities reciprocally. But the velocities reciprocally are as the times,
and therefore the times directly and the velocities reciprocally are as the
squares of the times; and therefore the quantities of matter are as the mo¬
tive forces and the squares of the times, that is, as the weights and the
squares of the times. Q.E.D.

Cor. 1. Therefore if the times are equal, the quantities of matter in
each of the bodies are as the weights.

Cor. 2. If the weights are equal, the quantities of matter will be as the
- squares of the times.

Cor. 3. If the quantities of matter are equal, the weights will berecip-
locally as the squares of the times.

Cor. 4. Whence since the squares of the times, cceteris paribus , are as
the lengths- of the pendulums, therefore if both the times and quantities of
matter are equal, the weights will be as the lengths of the pendulums.

J04

THE MATHEMATICAL PRINCIPLES

[Book il

Cor. 5. And universally, the quantity of matter in the pendulous body
is as the weight and the square of the time directly, and the length of the
pendulum inversely.

Cor. 6. But in a non-resisting medium, the quantity of matter in the
pendulous body is as the comparative weight and the square of the time
directly, and the length of the pendulum inversely. For the comparative
weight is the motive force of the body in any heavy medium, as was shewn
above; and therefore does the same thing in such a non-resisting medium
as the absolute weight does in a vacuum.

Cor. 7. And hence appears a method both of comparing bodies one
among another, as to the quantity of matter in each ; and of comparing
the weights of the same body in different places, to know the variation of
its gravity. And by experiments made with the greatest accuracy, I
have always found the quantity of matter in bodies to be proportional to
their weight.

PROPOSITION XXV. THEOREM XX.
Funependulous bodies that are, in any medium , resisted in the ratio of
the moments of time , and funependulous bodies that move in a non¬
resisting medium of the same specific gravity , perform their oscilla¬
tions in a cycloid in the same time, and describe proportional parts of
arcs together.

Let AB be an arc of a cycloid, which
a body D, by vibrating in a non-re¬
sisting medium, shall describe in any
time. Bisect that arc in C, so that C
may be the lowest point thereof; and
the accelerative force with which the
body is urged in any place D, or d or
E, will be as the length of the arc CD,
or C d, or CE. Let that force be ex¬
pressed by that same arc; and since the resistance is as the moment of the
time, and therefore given, let it be expressed by the given part CO of the
cycloidal arc, and take the arc O d in the same ratio to the arc CD that
the arc OB has to the arc CB : and the force with which the body in d is
urged in a resisting medium, being.the excess of the force C d above the
resistance CO, will be expressed by the arc Od, and will therefore be to
the force with which the body D is urged in a non-resisting medium in the
place D, as the arc Od to the arc C D ; and therefore also in the place B,
as the arc OB to the arc CB. Therefore if two bodies D, d go from the place
B, and are urged by these forces; since the forces at the beginning are as
the arc CB and OB, the first velocities and arcs first described will be in
the same ratio. Let those arcs be BD and B d f and the remaining arc*

Sec. YI.|

OF NATURAL PHILOSOPHY.

305

CD, Oclj will be in the same ratio. Therefore the forces, being propor¬
tional to those arcs CD, O d, will remain in the same ratio as at the be¬
ginning, and therefore the bodies will continue describing together arcs in
the same ratio. Therefore the forces and velocities and the remaining arcs
CD. Od, will be always as the whole arcs CB, OB, and therefore those re-
maininsr arcs wi.l be described together. Therefore the two bodies D and
d will arrive together at the places C and O ; that whicli moves in the
non-resisting medium, at the place C, and the other, in the resisting me¬
dium, at the place O. Now since the velocities in C and O areas the arcs
CB, OB, the arcs which the bodies describe when they go farther will be
in the same ratio. Let those arcs be CE and Oe. The force with which
the body D in a non-resisting medium is retarded in E is as CE, and the
force with which the body d in the resisting medium is retarded in e, is as
the sum of the force Ce and the resistance CO, that is, as Oe; and there¬
fore the forces with which the bodies are retarded are as the arcs CB, OB,
proportional to the arcs CE, Oe; and therefore the velocities, retarded in
that given ratio, remain in the same given ratio.' Therefore the velocities
and the arcs described with those velocities are always to each other in
that oriven ratio of the arcs CB and OB ; and therefore if the entire arcs
AB, aB are taken in the same ratio, the bodies 1) andd will describe those
aics together, and in the places A and a will lose all their motion together.
Therefore the whole oscillations are isochronal, or are performed in equal
times ; and any parts of the arcs, as BD, B d, or BE, Be, that are described
together, are proportional to the whole arcs BA, B a. Q.E.D.

Cor. Therefore the swiftest motion in a resisting medium does not fall
upon the lowest point C, but is found in that point 0, in which the whole
arc described Ba is bisected. And the body, proceeding from thence to a,
is retarded at the same rate with which it was accelerated before in its de¬
scent from B to O.

PROPOSITION XXYI. THEOREM XXL

Funependulous bodies , that are resisted in the ratio of the velocity, have
their oscillatio?is in a cycloid isochronal .

For if two bodies, equally distant from their centres of suspension, de¬
scribe, in oscillating, unequal arcs, and the velocities in the correspondent
parts of the arcs be to each other as the whole arcs; the resistances, pro¬
portional to the velocities, will be also to each other as the same arcs.
Therefore if these resistances be subducted from or added to the motive
forces arising from gravity which are as the same arcs, the differences or
sums will be to each other in the same ratio of the arcs; and since the in¬
crements and decrements of the velocities are as these differences or sums,
the velocities will be always as the whole arcs; therefore if the velocities
are in any one case as the whole arcs, they will remain always in the same

20

306

THE MATHEMATICAL PRINCIPLES

[Book. 1J

ratio. But at the beginning of the motion, when the bodies begin to de¬
scend and describe those arcs, the forces, which at that time are proportional
to the arcs, will generate velocities proportional to the arcs. Therefore
the velocities will be always as the whole arcs to be described, and there¬
fore those arcs wfill be described in the same time. Q.E.D.

PROPOSITION XXVII. THEOREM XXII.

Ij funependulons bodies are resisted in the duplicate ratio of their
velocities , the differences between, the times of the oscillations in a re¬
sisting medium, and the times of the oscillations in a non-resisting
medium of the same specific gravity , will be proportional to the arcs
described in oscillntims nearly.

For let equal pendulums in a re¬
sisting medium describe the unequal
arcs A, B ; and the resistance of the
body in the arc A will be to the resist¬
ance of the body in the correspondent
part of the arc B in the duplicate ra¬
tio of the velocities, that is, as, A A to
BB nearly. If the resistance in the
arc B were to the resistance in the arc
A as AB to A A, the times in the arcs A and B would be equal (by the last
Prop.) Therefore the resistance AA in the arc A, or AB in the arc B,
causes the excess of the time in the arc A above the time in a non-resisting
medium; and the resistance BB causes the excess of the time in the arc B
above the time in a non-resisting medium. But those excesses are as the

efficient forces AB and BB nearly, that is, as the arcs A and B. Q..E.D.

Cor. 1. Hence from the times of the oscillations in unequal arcs in a
resisting medium, may be known the times of the oscillations in a non-re¬
sisting medium of the same specific gravity. For the difference of the
times will be to the excess of the time in the lesser arc above the time in a
non-resisting medium as the difference of the arcs to the lesser arc.

Cor. 2. The shorter oscillations are more isochronal, and very short
ones are performed nearly in the same times as in a non-resisting medium.
But the times of those which are performed in greater arcs are a little
greater, because the resistance in the descent of the body, by which the
time is prolonged, is greater, in proportion to the length described in the
descent than the resistance in the subsequent ascent, by which the time is
contracted. But the time of the oscillations, both short and long, seems to
be prolonged in some measure by the motion of the medium. For retard¬
ed bodies are resisted somewhat less in proportion to the velocity, and ac¬
celerated bodies somewhat more than those that proceed uniformly forwards;

OF NATURAL PHILOSOPHY.

307

Sec. VI.]

because the medium, by the motion it has received from the bodies, going
forwards the same way with them, is more agitated in the former case, and
less in the latter; and so conspires more or less with the bodies moved.
Therefore it resists the pendulums in their descent more, and in their as¬
cent less, than in proportion to the velocity; and these two causes concur¬
ring prolong the time.

PROPOSITION XXVIII. THEOREM XXIII.

If a funependulous body , oscillating in a cycloid , be resisted in the rati >
of the moments of the time , its resistance will be to the force of grav¬
ity as the excess of the arc described in the whole descent above the
arc described in the subsequent ascent to twice the length of the pen¬
dulum.

Let BO represent the arc described
in the descent, C a the arc described in
the ascent, and A a the difference of
the arcs : and things remaining as they
were constructed and demonstrated in
Prop. XXV, the force with which the
oscillating body is urged in any place
D will be to the force of resistance as
the arc CD to the arc CO, which is
half of that difference A a. Therefore the force with which the oscillating
body is urged at the beginning or the highest point of the cycloid, that is,
the force of gravity, will be to the resistance as the arc of the cycloid, be¬
tween that highest point and lowest point C, is to the arc CO; that is
(doubling those arcs), as the whole cycloidal arc, or twice the length of the
pendulum, to the arc A a. Q.E.D.

PROPOSITION XXIX. PROBLEM VI.

Supposing that a body oscillating in a. cycloid is resisted in a duplicate
ratio of the velocity: to find the resistance in each place .

Let Ba be an arc described in one entire oscillation, C the lowest point

o s p rR Q

of the cycloid, and CZ half the whole cycloidal arc, equal to the length of
the pendulum ; and let it be required to find the resistance of the body is

309

THE MATHEMATICAL PRINCIPLES

[Book 1L

any place D. Cut the indefinite right line OQ in the points O, S, P, Q,
so that (erecting the perpendiculars OK ; ST, PI, QE, and with the centre
O, and the aysmptotes OK, OQ, describing the hyperbola TIGE cutting
the perpendiculars ST, PI, QE in T, I, and E, and through the point I
drawing KF, parallel to the asymptote OQ, meeting the asymptote OK i i
K, and the perpendiculars ST and QE in L and F) the hyperbolic area
PIEQ may be to the hyperbolic area PITS as the arc BC, described in the
descent of the body, to the arc C a described in the ascent; and that the
area IEF may be to the area ILT as OQ to OS. Then with the perpen¬
dicular MN cut off the hyperbolic area PINM, and let that area be to the
hyperbolic area PIEQ as the arc CZ to the arc BC described in the de¬
scent. And if the perpendicular RG cut off the hyperbolic area PIGR,
which shall be to the area PIEQ as any arc CD to the arc BC described
in the whole descent, the resistance in any place D will be to the force of

gravity as the area

OR

oq iep

IGH to the area PINM.

For since the forces arising from gravity with which the body is
urged in the places Z, B, D, a, are as the arcs CZ, CB, CD, C a and those
arcs are as the areas PINM, PIEQ, PIGR, PITS; let those areas be the
exponents both of the arcs and of the forces respectively. Let D d be a
very small space described by the body in its descent: and let it be expressed
by the very small area RGgr comprehended between the parallels RG, rg ;
and produce rg* to //, so that GYihg and RGgr may be the contemporane¬
ous decrements of the areas IGH, PIGR. A.nd the increment Gllhg —

Rr IEF, or RrxHG-^ IEF, of the area ~ IEF — IGH will be

OQ

OQ

OQ

IEF

OQ

to the decrement RGgr, or Rr X RG, of the area PIGR, as HG —

OR

to RG ; and therefore as OR X HG — IEF to OR X OR or OP X

PI, that is (because of the equal quantities OR X HG, OR X HR — OR
X GR, ORHK — OPIK, PIHR and PIGR + IGH), as PIGR + IGH —

IEF to OPIK. Therefore if the area IEF — IGH he called

Y, and RGgr the decrement of the area PIGR be given, the increment of
the area Y will be as PIGR — Y.

Then if V represent the force arising from the gravity, proportional to
the arc CD to be described, by which the body is acted upon in D, and R
be put for the resistance, Y — R will be the whole force with which the
body is urged in D. Therefore the increment of the velocity is as Y — R
and the particle of time in which it is generated conjunctly. But the ve¬
locity itself is as the contempoi aneous increment of the space described di-

Sec. VI.]

OF NATURAL PHILOSOPHY.

309

rectly and the same particle of time inversely. Therefore, since the re¬
sistance is, by the supposition, as the square of the velocity, the increment
of the resistance will (by Lem. II) be as the velocity and the increment of
the velocity conjunctly, that is, as the moment of the space and V — R
conjunctly ; and, therefore, if the moment of the space be given, as V —
R; that is, if for the force V we put its exponent PIGR, and the resist¬
ance R be expressed by any other area Z, as PIGR — Z.'

Therefore the area PIGR uniformly decreasing by the subduction of
given moments, the area Y increases in proportion of PIGR — Y, and
the area Z in proportion of PIGR — Z. And therefore if the areas
Y and Z begin together, and at the beginning are equal, these, by the
addition of equal moments, will continue to be equal; and in like man¬
ner decreasing by equal moments, will vanish together. And, vice versa ,
if they together begin and vanish, they will have equal moments and be
always equal; and that, because if the resistance Z be augmented, the ve¬
locity together with the arc C a, described in the ascent of the body, will be
diminished; and the point in which all the motion together with the re¬
sistance ceases coming nearer to the point C, the resistance vanishes sooner
than the area Y. And the contrary will happen when the resistance is
diminished.

Now the area Z begins and end^s where the resistance is nothing, that is,
at the beginning of the motion where the arc CD is equal to the arc CB,

and the right line RG falls upon the right line QE; and at the end of
the motion where the arc CD is equal to the arc C a, and RG falls upon

OR

the right line ST. And the area'Y or 7 -pr IEF— IGH begins and ends

also where the resistance is nothing, and therefore where

OR

OQ,

IEF

and

IGH are equal; that is (by the construction), where the right line RG
falls successively upon the right lines QE and ST. Therefore those areas
begin and vanish together, and are therefore always equal. Therefore the area
OR

IEF — IGH is equal to the area Z, by which the resistance is ex¬

pressed, and therefore is to the area PINM, by which the gravity is ex¬
pressed, as the resistance to the gravity. Q.E.D.

310 THE MATHEMATICAL PRINCIPLES [BOOK 11.

Cor. 1 . Therefore the resistance in the lowest place C is to the force
OP

of gravity as the area IEF to the area PINM.

Cor. 2. But it becomes greatest where the area PIHR is to the area
IEF as OR to OQ,. For in that case its moment (that is, PIGR — Y)
becomes nothing.

Cor. 3. Hence also may be known the velocity in each place, as being
in the subduplicate ratio of the resistance, and at the beginning of the mo¬
tion equal to the velocity of the body oscillating in the same cycloid with¬
out any resistance.

However, by reason of the difficulty of the calculation by which the re¬
sistance and the velocity are found by this Proposition, we have thought
fit to subjoin the Proposition following.

PROPOSITION XXX. THEOREM XX1Y.

If a right Urn aB be equal to the arc of a cycloid which an oscillating
body describes, and at each of its points D the perpendiculars DK be
erected, which shall be to the length of the pendulum as the resistance
of the body in the corresponding points of the arc to the force of grav¬
ity ; I say, that the difference between the arc described in the ivhole
descent and the arc described in the whole subsequent ascent drawn
into half the sum of the same arcs will be equal to the area BKa
which all those perpendiculars take up.

Let the arc of the cycloid, de¬
scribed in one entire oscillation, be
expressed by the right line aB,
equal to it, and the arc which
would have been described in vacuo
by the length AB. Bisect AB in
C, and the point C will represent

CD will be as the force arising from gravity, with which the body in D is
urged in the direction of the tangent of the cycloid, and will have the same
ratio to the length of the pendulum as the force in D has to the force of
gravity. Let that force, therefore, be expressed by that length CD, and
the force of gravity by the length of the pendulum; and if in DE you
take DK in the same ratio to the length of the pendulum as the resistance
has to the gravity, DK will be the exponent of the resistance. From the
centre C with the interval CA or CB describe a semi-circle BEeA. Let
the body describe, in the least time, the space D d ; and, erecting the per¬
pendiculars DE, de, meeting the circumference in E and e, they will be as
the velocities which the body descending in vacuo from the point B would
acquire in the places D and d. This appears by Prop. LII, Book L Let

OF NATURAL PHILOSOPHY.

311

Sec. VI.]

therefore, these velocities be expressed by those perpendiculars DE, de ;
and let DF be the velocity which it acquires in D by falling from B in
the resisting medium. And if from the centre C with the interval OF we
describe the circle F/*M meeting the right lines de and AB in f and M,
then M will be the place to which it would thenceforward, without farther
resistance, ascend, and df the velocity it would acquire in d. Whence,
also, if Fff represent the moment of the velocity which the body D, in de¬
scribing the least space D</, loses by the resistance of the medium; and
CN be taken equal to Cg*; then will N be the place to which the body, if
it met no farther resistance, would thenceforward ascend, and MN will be
the decrement of the ascent arising from the loss of that velocity. Draw
F m perpendicular to df \ and the decrement F«- of the velocity DF gener¬
ated by the resistance DK will be to the increment//?! of the same velo¬
city. generated by the force CD, as the generating force DK to the gener¬
ating force CD. But because of the similar triangles F/nf F hg, FDC,
fm is to F m or Dd as CD to DF; and, ex ceqtio , F«* to F)d as DK to
DF. Also F h is to Fg- as DF to CF ; and, ex ccqun perturbatp , FA or
MN to F)d as DK to CF or CM ; and therefore the sum of all the MN X
CM will be equal to the sum of all the D d X DK. At the moveable
point M suppose always a rectangular ordinate erected equal to the inde¬
terminate CM, which by a continual motion is drawn into the whole
length A a ; and the trapezium described by that motion, or its equal, the
rectangle A a X |aB, will be equal to the sum of all the MN X CM, and
therefore to the sum of all the Fid X DK, that is, to the area BKVTa
Q.E.D.

Cor. Hence from the law of resistance, and the difference A a of the
arcs Ca, CB, may be collected the proportion of the resistance to the grav¬
ity nearly.

For if the resistance DK be uniform, the figure BKTa will be a rec¬
tangle under B a and DK; and thence the rectangle under |B a and A a
will be equal to the rectangle under Ba and DK, and DK will be equal to
IA a. Wherefore since DK is the exponent of the resistance, and the
length of the pendulum the exponent of the gravity, the resistance will be
to the gravity as a to the length of the pendulum ; altogether as in
Prop. XXVIII is demonstrated.

If the resistance be as the velocity, the figure BKTa will be nearly an
ellipsis. For if a body, in a non-resisting medium, by one entire oscilla¬
tion, should describe the length BA, the velocity in any place D would be
as the ordinate DE of the circle described on the diameter AB. There¬
fore since Ba in the resisting medium, and BA in the non-resisting one,
are described nearly in the same times; and therefore the velocities in each
of the points of Ba are to the velocities in the correspondent points of the
length BA nearly as Ba is to BA, the velocity in the point D in the re-

312

THE MATHEMATICAL PRINCIPLES

[B .)0K 11.

sisting medium will be as the ordinate of the circle or ellipsis described
upon the diameter B a ; and therefore the figure BKVTa will be nearly ac
ellipsis. Since the resistance is supposed proportional to the velocity, le<.
OV be the exponent of the resistance in the middle point O; and an ellip¬
sis BRVSa described with the centre O, and the semi-axes OB, OY, will
be nearly equal to the figure BKVTa, and to its equal the rectangle A a
X BO. Therefore A a X BO is to OY X BO as the area of this ellipsis
to OY X BO; that is, A a is to OY as the area of the semi-circle to the
square of the radius, or as 11 to 7 nearly ; and, therefore, T 7 T Aa is to the
length of the pendulum as the resistance of the oscillating body in O to
its gravity.

Now if the resistance DK be in the duplicate ratio of the velocity, the
figure BKVTa will be almost a parabola having Y for its vertex and OV
for its axis, and therefore will be nearly equal to the rectangle under |B a
and OY. Therefore the rectangle under |Ba and A a is equal to the rec¬
tangle §B a X OY, and therefore OY is equal to £Aa; and therefore the
resistance in O made to the oscillating body is to its gravity as f A a to the
length of the pendulum.

And I take these conclusions to be accurate enough for practical uses.
For since an ellipsis or parabola BRVSa falls in with the figure BKVTa
in the middle point Y, that figure, if greater towards the part BRY or
YS a than the other, is less towards the contrary part, and is therefore
nearly equal to it.

PROPOSITION XXXI. THEOREM XXY.

If the 1 'esistance made to cm oscillating- body in each of the proportional
parts of the arcs described be augmented or diminished in. a given ra¬
tio, the difference between the arc described in the descent and the arc
described in the subsequent ascent will be augmented or diminished in
the same ratio .

For that difference arises from
the retardation of the pendulum
by the resistance of the medium,
and therefore is as the w r hole re¬
tardation and the retarding resist-
ance proportional thereto. In the
foregoing Proposition the rectan-

the difference Aa of the arcs CB, Ca, was equal to the area BKTa. And
that area, if the length aB remains, is augmented or diminished in the ra¬
tio of the ordinates DK; that is, in the ratio of the resistance and is there¬
fore as the length aB and the resistance conjunctly. And therefore the
rectangle under A a and £«B is as aB and the resistance conjunctly, anc
therefore Aa is as the resistance. Q,.E.D.

Sec. YI.1

OF NATURAL PHILOSOPHY.

313

Cor. 1. Hence if the resistance be as the velocity, the difference of
the arcs in the same medium will be as the whole arc described: and the
contrary.

Cor. 2. If the resistance be in the duplicate ratio of the velocity, that
difference will be in the duplicate ratio of the whole arc : and the contrary.

Cor. 3. And universally, if the resistance be in the triplicate or any
other ratio of the velocity, the difference will be in the same ratio of the
whole arc : and the contrary.

Cor. 4. If the resistance be partly in the simple ratio of the velocity,
and partly in the duplicate ratio of the same, the difference will be partly
in the ratio of the whole arc, and partly in the duplicate ratio of it: and
the contrary. So that the law and ratio of the resistance will be the
same for the velocity as the law and ratio of that difference for the length
of the arc.

Cor. 5. And therefore if a pendulum describe successively unequal arcs,
and we can find the ratio of the increment or decrement of this difference
for the length of the arc described, there will be had also the ratio of the
increment or decrement of the resistance for a greater or less velocity.

GENERAL SCHOLIUM.

From these propositions we may find the resistance of mediums by pen¬
dulums oscillating therein. I found the resistance of the air by the fol¬
lowing experiments. I suspended a wooden globe or ball weighing 57^
ounces troy, its diameter Gf London inches, by a fine thread on a firm
hook, so that the distance between the hook and the centre of oscillation of
the globe was 10| feet. I marked on the thread a point 10 feet and 1 inch
distant from the centre of suspension; and even with that point I placed a
ruler divided into inches, by the help whereof I observed the lengths of the
arcs described by the pendulum. Then I numbered the oscillations in
which the globe would lose { part of its motion. If the pendulum was
drawn aside from the perpendicular to the distance of 2 inches, and thence
let go, so that in its whole descent it described an arc of 2 inches, and in
the first whole oscillation, compounded of the descent and subsequent
ascent, an arc of almost 4 inches, the same in 164 oscillations lost j part
of its motion, so as in its last ascent to describe an arc of If inches. If
in the first descent it described an arc of 4 inches, it lost { part of its mo¬
tion in 121 oscillations, so as in its last ascent to describe an arc of 3|
inches. If in the first descent it described an arc of 8,16,32, or 64 inches,
it lost | part of its motion in 69, 35|, 18j, 9f oscillations, respectively.
Therefore the difference between the arcs described in the first descent and
the last ascent was in the 1st, 2d, 3d, 4th, 5th, 6th cases, 1, 2, 4, 8
inches respectively. Divide those differences by the number of oscillations
in each case, and in one mean oscillation, wherein an arc of 3f, 7|, 15, 30

314

THE MATHEMATICAL PRINCIPLES

[Book IJ.

60 ; 120 inches was described, the difference of the arcs described in the
descent and subsequent ascent will be t 4 t . §£ parts of an

inch, respectively. But these differences in the greater oscillations are in
the duplicate ratio of the arcs described nearly, but in lesser oscillations
something greater than in that ratio ; and therefore (by Cor. 2, Prop. XXXI
of this Book) the resistance of the globe, when it moves very swift, is in
the duplicate ratio of the velocity, nearly; and when it moves slowly,
somewhat greater than in thftt ratio.

Now let V represent the greatest velocity in any oscillation, and let A,
B, and C be given quantities, and let us suppose the difference of the arcs

3

to be AY + BY 2 + CY 2 . Since the greatest velocities are in the cycloid
as ^ the arcs described in oscillating, and in the circle as \ the chords of
those arcs; and therefore in equal arcs are greater in the cycloid than in
the circle in the ratio of J the arcs to their chords; but the times in the
circle are greater than in the cycloid, in a reciprocal ratio of the velocity;
it is plain that the differences of the arcs (which are as the resistance and
the square of the time conjunctly) are nearly the same in both curves: for
in the cycloid those differences must be on the one hand augmented, with
the resistance, in about the duplicate ratio of the arc to the chord, because
of the velocity augmented in the simple ratio of the same; and on the
other hand diminished, with the square of the time, in the same duplicate
ratio. Therefore to reduce these observations to the cycloid, we must take
the same differences of the arcs as were observed in the circle, and suppose
the greatest velocities analogous to the half, or the whole arcs, that is, to
the numbers i, 1, 2, 4, 8, 16. Therefore in the 2d, 4th, and 6th cases, put
1,4, and 16 for Y; and the difference of the arcs in the 2d case will become

i 2

—= A -f B + C; in the 4th case, = 4A + 8B + 16C ; in the 6th

121 o&j

case, ^- = 16A -f 64B -f- 256C. These equations reduced give A =

0,0000916, B = 0,0010847, and C = 0,0029558. Therefore the difference

of the arcs is as 0,0000916V + 0,0010847V 5 + 0,0029558V*: and there¬
fore since (by Cor. Prop. XXX, applied to this case) the distance of the
globe in the middle of the arc described in oscillating, where the velocity

is Y, is to its weight as T 7 T AV + T 7 „BV 2 + fCV 2 to the length of the
pendulum, if for A, B, and C you put the numbers found, the resistance of

the globe will be to its weight as 0,0000583V + 0,0007593V^ + 0,0022169Y 2
to the length of the pendulum between the centre of suspension and the
ruler, that is, to 121 inches. Therefore since Y in the second case repre¬
sents 1, in the 4th case 4, and in the 6th case 16, the resistance will be to
the weight of the globe, in the 2d case, as 0,0030345 to 121; in the 4th, as
0,041748 to 121; in the 6th, as 0,61705 to 121.

315

Sec. VI.] of natural philosophy.

The arc, which the point marked in the thread described in the 6 th case,

was of 120 — or 119^ inches. And therefore since the radius was

121 inches, and the length of the pendulum between the point of suspen¬
sion and the centre of the globe was 126 inches, the arc which the centre of
the globe described was 124/ T inches. Because the greatest velocity of the
oscillating body, by reason of the resistance of the air, does not fall on the
lowest point of the arc described, but near the middle place of the whole
arc, this velocity will be nearly the same as if the globe in its whole descent
in a non-resisting medium should describe 62g\ inches, the half of that arc,
and that in a cycloid, to which we have above reduced the motion of the
pendulum; and therefore that velocity will be equal to that which the
globe would acquire by falling perpendicularly from a height equal to the
versed sine of that arc. But that versed sine in the cycloid is to that arc
62/2 as the same arc to twice the length of the pendulum 252, and there¬
fore equal to 15,278 inches. Therefore the velocity of the pendulum is the
same which a body would acquire by falling, and in its fall describing a
space of 15,278 inches. Therefore with such a velocity the globe meets
with a resistance which is to its weight as 0,61705 to 121, or (if we take
that part only of the resistance which is in the duplicate ratio of the ve¬
locity) as 0,56752 to 121.

I found, by an hydrostatical experiment, that the weight of this wooden
globe was to the weight of a globe of water of the same magnitude as 55
to 97: and therefore since 121 is to 213,4 in the same ratio, the resistance
made to this globe of water, moving forwards with the above-mentioned
velocity, will be to its weight as 0,56752 to 213,4, that is, as 1 to 376^.
Whence since the weight of a globe of water, in the time in which the
globe with a velocity uniformly continued describes a length of 30,556
inches, will generate all that velocity in the falling globe, it is manifest
that the force of resistance uniformly continued in the same time will take
away a velocity, which will be less than the other in the ratio of 1 to 376^- 0 ,

that is, the —part of the whole velocity. And therefore in the time

iiat the globe, with the same velocity uniformly continued, would describe
the length of its semi-diameter, or 3 r 7 F inches, it would lose the 33 V 2 P ar ^
of its motion.

I also counted the oscillations in which the pendulum lost | part of its
motion. In the following table the upper numbers denote the length of the
arc described in the first descent, expressed in inches and parts of an inch;
the middle numbers denote the length of the arc described in the last as¬
cent ; and in the lowest place are the numbers of the oscillations. I give
an account of this experiment, as being more accurate than that in which

316 THE MATHEMATICAL PRINCIPLES [BOOK ll

only i part of the motion was lost. I leave the calculation to such as are
disposed to make it.

First descent . .

. 2

4

8

16

32

64

Last ascent . .

• 4

3

6

12

24

.48

Numb . of oscill. .

. 374

272

162i

83i

4l|

22i

I afterward suspended a leaden globe of 2 inches in diameter, weighing
26i ounces troy by the same thread, so that between the centre of the
globe and the point of suspension there was an interval of 10i feet, and I
counted the oscillations in which a given part of the motion was lost. The
lirst of the following tables exhibits the number of oscillations in which -J-
part of the whole motion was lost; the second the number of oscillations
in which there was lost i part of the same.

First descent . .

. . 1

2

4

8

16

32

64

Last ascent . .

I

7

4

3!

7

14

28

56

Numb, of oscill.

. . 226 *

228

193

140

90i

53

30

First descent . .

. . 1

2

4

8

16

32

64

Last ascent . 4

3

* * 4

4

3

6

12

24

4S

Numb, of oscill .

. . 510

518

420

318

204

121

70

Selecting in the first table the 3d, 5th, and 7th observations, and express¬
ing the greatest velocities in these observations particularly by the num¬
bers 1, 4, 16 respectively, and generally by the quantity Y as above, there

A

will come out in the 3d observation = A -f B + C, in the 5th obser-

2 8

vation = 4A 4- SB + 16C, in the 7th observation = 16A + 64B -t-

256C. These equations reduced give A == 0,001414, B = 0,000297, C =-
0,000879. And thence the resistance of the globe moving with the velocity
V will be to its weight 26} ounces in the same ratio as 0,0009V +

0,000208V 2 + 0,000659V 2 to 121 inches, the length of the pendulum.
And if we regard that part only of the resistance which is in the dupli¬
cate ratio of the velocity, it will be to the weight of the globe as 0,000659V 2
to 121 inches. But this part of the resistance in the first experiment was
to the weight of the wooden globe of 57 g 7 j ounces as 0,002217V 2 to 121;
and thence the resistance of the wmoden globe is to the resistance of the
leaden one (their velocities being equal) as 57into 0,002217 to 26J-
into 0,000659, that is, as 71 to 1. The diameters of the two globes were
6J and 2 inches, and the squares of these are to each other as 47! and 4,
or 11 if and 1, nearly. Therefore the resistances of these equally swift
globes were in less than a duplicate ratio of the diameters. But we have
not yet considered the resistance of the thread, which was certainly very
considerable, and ought to be subducted from the resistance of the pendu¬
lums here found. I could not determine this accurately, but I found ii

OF NATURAL PHILOSOPHY.

31/

Sec. VI.J

greater than a third part of the whole resistance of the lesser pendulum ;
and thence I gathered that the resistances of the globes, when the resist¬
ance of the thread is subducted, are nearly in the duplicate ratio of their
diameters. For the ratio of 7} — } to 1 — }, or 10} to 1 is not very
different from the duplicate ratio of the diameters 1 l}f to l.

Since the resistance of the -thread is of less moment in greater globes, I
tried the experiment also with a globe whose diameter was 3Sf inches.
The length of the pendulum between the point of suspension and the cen¬
tre uf oscillation was 12 2} inches, and between the point of suspension and
the knot in the thread 109} inches. The arc described by the knot at the
first descent of the pendulum was 32 inches. The arc described by the
same knot in the last ascent after five oscillations was 2S inches. The
sum of the arcs, or the whole arc described in one mean oscillation, was 60
inches. The difference of the arcs 4 inches. The T V part of this, or the
difference between the descent and ascent in one mean oscillation, is f of
an inch. Then as the radius 109 } to the radius 122}, so is the whole arc
of 60 inches described by the knot in one mean oscillation to the whole arc
of 67} inches described by the centre of the globe in one mean oscillation;
and so is the difference § to a new difference 0,4475. If the length of the
arc described were to remain, and the length of the pendulum should be
augmented in the ratio of 126 to 122}, the time of the oscillation would
be augmented, and the velocity of the pendulum would be diminished in
the subduplicate of that ratio ; so that the difference 0,4475 of the arcs de¬
scribed in the descent and subsequent ascent would remain. Then if the
arc described be augmented in the ratio of 124/ T to 67}, that difference
0.4475 would be augmented in the duplicate of that ratio, and so would
become 1,5295. These things would be so upon the supposition that the
resistance of the pendulum were in the duplicate ratio of the velocity.
Therefore if the pendulum describe the whole arc of 1243 3 T inches, and its
length between the point of suspension and the centre of oscillation be 126
inches, the difference of the arcs described in the descent and subsequent
ascent would be 1,5295 inches. And this difference multiplied into the
weight of the pendulous globe, which was 208 ounces, produces 318,136.
Again; in the pendulum above-mentioned, made of a wooden globe, when
its centre of oscillation, being 126 inches from the point of suspension, de¬
scribed the whole arc of 124/ r inches, the difference of the arcs described

126 8

in the descent and ascent was into This multiplied into the

weight of the globe, which was 67^ ounces, produces 49,396. But I mul¬
tiply these differences into the weights of the globes, in order to find their
resistances. For the differences arise from the resistances, and are as the
resistances directly and the weights inversely. Therefore the resistances
are as the numbers 318,136 and 49,396. But that part of the resistance

THE MATHEMATICAL PRINCIPLES

[Book IL

31S

of the lesser globe, which is in the duplicate ratio of the velocity, was to
the whole resistance as 0,56752 tor 0,61675, that is, as 45,453 to 49,396;
w'hereas that part of the resistance of the greater globe is almost equal to
its whole resistance; and so those parts are nearly as 318,136 and 45,453,
that is, as 7 and 1. But the diameters of the globes are 18f and 6J; and
their squares 351 y 9 ^ and 47ii are as 7,438 and 1, that is, as the resistances
of the globes 7 and 1, nearly. The difference of these ratios is scarce
greater than may arise from the resistance of the thread. Therefore those
parts of the resistances which are, when the globes are equal, as the squares
of the velocities, are also, when the velocities are equal, as the squares of
the diameters of the globes.

But the greatest of the globes I used in these experiments was not per¬
fectly spherical, and therefore in this calculation I have, for brevity’s sake,
neglected some little niceties; being not very solicitous for an accurate
calculus in an experiment that was not very accurate. So that I could
wish that these experiments were tried again with other globes, of a larger
size, more in number, and more accurately formed; since the demonstra¬
tion of a vacuum depends thereon. If the globes be taken in a geometrical
proportion, as suppose whose diameters are 4, 8, 16, 32 inches ; one may
collect from the progression observed in the experiments what would hap¬
pen if the globes were still larger.

In order to compare the resistances of different fluids with each other, I
made the following trials. I procured a wooden vessel 4 feet long, 1 foot
broad, and 1 foot high. This vessel, being uncovered, I filled with spring
water, and, having immersed pendulums therein, I made them oscillate in
the water. And I found that a leaden globe weighing 166i ounces, and in
diameter 3f inches, moved therein as it is set down in the following table;
the length of the pendulum from the point of suspension to a certain
point marked in the thread being 126 inches, and to the centre of oscilla¬

tion 134f inches.

The arc described in'
the first descent , by

a point marked in J-64 . 32 . 16 . 8 . 4

the thread was

inches.

The arc described in )
the last ascent was
inches.

The difference of the
arcs, proportional
to the motion lost ,
was inches .

The number of the os¬
cillations in water.

The number of the os¬
cillations in air.

48 . 24 . 12 . 6

16

2 9
60

851

. 287.535

. 2 . 1 . £ . J

• li • I • i • A

-*-*.*.*
. 7 . 11J.12|.13*

OF NATURAL PHILOSOPHY.

319

Sec. VI.]

In the experiments of the 4th column there were equal motions lost in
535 oscillations made in the air, and Ifin water. The oscillations in the
air were indeed a little swifter than those in the water. But if the oscil¬
lations in the water were accelerated in such a ratio that the motions of
the pendulums might be equally swift in both mediums, there would be
still the same number 1 j of oscillations in the water, and by these the
same quantity of motion would be lost as before ; because the resistance b
increased, and the square of the time diminished in the same duplicate ra¬
tio. The pendulums, therefore, being of equal velocities, there were equal
motions lost in 535 oscillations in the air, and 1} in the water; and there¬
fore the resistance of the pendulum in the water is to its resistance in the
air as 535 to 1J. This is the proportion of the whole resistances in the
case of the 4th column.

Now let AY + CV 2 represent the difference of the arcs described in the
descent and subsequent ascent by the globe moving in air with the greatest
velocity Y; and since the greatest velocity is in the case of the 4th column
to the greatest velocity in the case of the 1st column as 1 to 8; and that
difference of the arcs in the case of the 4th column to the difference in the

2 16

case of the 1st column as to g^y, or as 85£ to 42S0 ; put in these

cases 1 and 8 for the velocities, and 85 \ and 4280 for the differences of
the arcs, and A + C will be = S5f, and 8A + 64C = 4280 or A + SC
= 535; and then by reducing these equations, there will come out 7C =
449| and C = 64 T 3 T and A = 21-f ; and therefore the resistance, which is
as T 7 T AY + fCY 2 , will become as 13 T 6 T Y + 48/gY 2 . Therefore in the
case of the 4th column, where the velocity was 1, the whole resistance is to
its part proportional to the square of the velocity as 13 T 6 T + 4S/ F or
61 to 48 j 9 f ; and therefore the resistance of the pendulum in water is to
that part of the resistance in air, which is proportional to the square of the
velocity, and which in swift motions is the only part that deserves consid¬
eration, as 61}f to 4 Sj 9 f and 535 to \\ conjunctly, that is, as 571 to 1.
If the whole thread of the pendulum oscillating in the water had been im¬
mersed, its resistance would have been still greater; so that the resistance
of the pendulum oscillating in the water, that is, that part which is pro¬
portional to the square of the velocity, and which only needs to be consid¬
ered in swift bodies, is to the resistance of the same whole pendulum, oscil¬
lating in air with the same velocity, a3 about 850 to 1, that is as, the den¬
sity of water to the density of air, nearly.

In this calculation we ought also to have taken in that part of the re¬
sistance of the pendulum in the water which was as the square of the ve¬
locity ; but I found (which will perhaps seem strange) that the resistance
in the water was augmented in more than a duplicate ratio of the velocity.
In searching after the cause, I thought upon this, that the vessel was toe

320

THE MATHEMATICAL PRINCIPLES

[Book IL

narrow for the magnitude of the pendulous globe, and by its narrowness
obstructed the motion of the water as it yielded to the oscillating globe.
For when I immersed a pendulous globe, whose diameter was one inch only,
the resistance was augmented nearly in a duplicate ratio of the velocity.
I tried this by making a pendulum of two globe 3 , of which the lesser and
lower oscillated in the water, and the greater and higher was fastened to
the thread just above the water, and, by oscillating in the air, assisted the
motion of the pendulum, and continued it longer. The experiments made
by this contrivance proved according to the following table.

Arc descr. in first descent ..16.S.4.2.1.1.J.
Arc descr. iri last ascent . . 12 . 6 . 3 . 1} . | . | . _ s ¥

Diff. of arcs , proport, to) - o 1 x L i _i_

motion lost $ * * ’ 2 * 4 * « *

Number of oscillations... 3f . 6} . 12^. 21}. 34 . 53 . 62)

In comparing the resistances of the mediums with each other, I also
caused iron pendulums to oscillate in quicksilver. The length of the iron
wire was about 3 feet, and the diameter of the pendulous globe about } of
an inch. To the wire, just above the quicksilver, there was fixed another
leaden globe of a bigness sufficient to continue the motion of the pendulum
for some time. Then a vessel, that would hold about 3 pounds of quick¬
silver, was filled by turns with quicksilver and common water, that, by
making the pendulum oscillate successively in these two different fluids, I
might find the proportion of their resistances; and the resistance of the
quicksilver proved to be to the resistance of water as about 13 or 14 to 1 ;
that is. as the density of quicksilver to the density of water. When I made
use of a pendulous globe something bigger, as of one whose diameter was
about } or f of an inch, the resistance of the quicksilver proved to be to
the resistance of the water as about 12 or 10 to 1. But the former experi¬
ment is more to be relied on, because in the latter the vessel was too nar¬
row in proportion to the magnitude of the immersed globe; for the vessel
ought to have been enlarged together with the globe. I intended to have
repeated these experiments with larger vessels, and in melted metals, and
other liquors both cold and hot; but I had not leisure to try all: and be¬
sides, from what is already described, it appears sufficiently that the resist¬
ance of bodies moving swiftly is nearly proportional to the densities of
the fluids in which they move. I do not say accurately; for more tena¬
cious fluids, of equal density, will undoubtedly resist more than those that
are more liquid; as cold oil more than warm, warm oil more than rain¬
water, and water more than spirit of wine. But in liquors, which are sen¬
sibly fluid enough, as in air, in salt and fresh water, in spirit of wine, of
turpentine, and salts, in oil cleared of its fseces by distillation and warmed,
in oil of vitriol, and in mercury, and melted metals, and any other such
like, that are fluid enough to retain for some time the motion impressed

Sec. YI.J

OF NATURAL PHILOSOPHY.

321

upon them by the agitation of the vessel, and which being poured out are
easily resolved into drops, I doubt not but the rule already laid down may
be accurate enough, especially if the experiments be made with larger
pendulous bodies and more swiftly moved.

Lastly, since it is the opinion of some that there is a certain aethereal
medium extremely rare and subtile, which freely pervades the pores of all
bodies; and from such a medium, so pervading the pores of bodies, some re¬
sistance must needs arise; in order to try whether the resistance, which we
experience in bodies in motion, be made upon their outward superficies only,
or whether their internal parts meet with any considerable resistance upon
their superficies, I thought of the following experiment I suspended a
round deal box by a thread 11 feet long, on a steel hook, by means of a ring
of the same metal, so as to make a pendulum of the aforesaid length. The
hook had a sharp hollow edge on its upper part, so that the upper arc of
the ring pressing on the edge might move the more freely; and the thread
was fastened to the lower arc of the ring. The pendulum being thus pre¬
pared, I drew it aside from the perpendicular to the distance of about 6
feet, and that in a plane perpendicular to the edge of the hook, lest the
ring, while the pendulum oscillated, .should slide to and fro on the edge of
the hook: for the point of suspension, in which the ring touches the hook,
ought to remain immovable. I therefore accurately noted the place to
which the pendulum was brought, and letting it go, I marked three other
places, to which it returned at the end of the 1st, 2d, and 3d oscillation.
This I often repeated, that I might find those places as accurately as pos¬
sible. Then I filled the box with lead and other heavy metals that were
near at hand. But, first, I weighed the box when empty, and that part of
the thread that went round it, and half the remaining part, extended be¬
tween the hook and the suspended box; for the thread so extended always
acts upon the pendulum, when drawn aside from the perpendicular, with half
its weight. To this weight I added the weight of the air contained in the
box And this whole weight was about of the weight of the box when
filled with the metals. Then because the box when full of the metals, by ex¬
tending the thread with its weight, increased the length of the pendulum,
\ shortened the thread so as to make the length of the pendulum, when os¬
cillating, the same as before. Then drawing aside the pendulum to the
place first marked, and letting it go, I reckoned about 77 oscillations before
the box returned to the second mark, and as many afterwards before it came
to the third mark, and as many after that before it came to the fourth
mark. From whence I conclude that the whole resistance of the box, when
full, had not a greater proportion to the resistance of the box, when empty,
than 78 to 77. For if their resistances were equal, the box, when full, by
reason of its vis insita, which was 78 times greater than the vis insita of
the same when emptv, ought to have continued its oscillating motion so

21

322

THE MATHEMATICAL PRINCIPLES

|Book II.

much the longer, and therefore to have returned to those marks at the end
of 78 oscillations. But it returned to them at the end of 77 oscillations.

Let, therefore, A represent the resistance of the box upon its external
superficies, and B the resistance of the empty box on its internal superficies;
and if the resistances to the internal parts of bodies equally swift be as the
matter, or the number of particles that are resisted, then 78B will be the
resistance made to the internal parts of the box, when full; and therefore
the whole resistance A + B of the empty box will be to the whole resist¬
ance A + 78B of the full box as 77 to 78, and, by division, A -f B to 77B
as 77 to 1; and thence A + B to B as 77 X 77 to 1, and, by division
again, A to B as 5928 to 1. Therefore the resistance of the empty box in
its internal parts will be above 5000 times less than the resistance on its
external superficies. This reasoning depends upon the supposition that the
greater resistance of the full box arises not from any other latent cause,
but only from the action of some subtile fluid upon the included metal.

This experiment is related by memory, the paper being lost in which I
had described it; so that I have been obliged to omit some fractional parts,
which are slipt out of my memory; and I have no leisure to try it again.
The first time I made it, the hook being weak, the full box was retarded
sooner. The cause I found to be, that the hook was not strong enough to
bear the weight of the box; so that, as it oscillated to and fro, the hook
was bent sometimes this and sometimes that way. I therefore procured a
hook of sufficient strength, so that the point of suspension might remain
unmoved, and then all things happened as is above described.

Sec. VII.1

OF NATURAL PHILOSOPHY.

323

SECTION VII.

Of the motion of fluids, and the resistance made to projected bodies .

PROPOSITION XXXII. THEOREM XXVI.

Suppose two similar systems of bodies consisting of an equal number oj
particles, and let the correspondent particles be similar and propor¬
tional, each in one system to each in the other, and have a like situa¬
tion among themselves, and the same given ratio of density to each
other ; and let them begin to move among themselves in proportional
times, and with like motions {that is, those in one system among one
another, and those in the other among one another). And if the par¬
ticles that are in the same system do not touch one another, except ir
the 'moments of reflexion ; nor attract, nor repel each other, except with
accelerative forces that are as the diameters of the correspondent parti¬
cles inversely, and the squares of the velocities directly ; I say, that the
particles of those systems will continue to move among themselves with
like motions and in proportional times.

Like bodies in like situations are »aid to be moved among themselves
with like motions and in proportional times, when their situations at the
end of those times are always found alike in respect of each other.; as sup¬
pose we compare the particles in one system with the correspondent parti¬
cles in the other. Hence the times will be proportional, in which similar
and proportional parts of similar figures will be described by correspondent
particles. Therefore if we suppose two systems of this kind, the corre¬
spondent particles, by reason of the similitude of the motions at their
beginning, will continue to be moved with like motions, so long as they
move without meeting one another; for if they are acted on by no forces,'
they will go on uniformly in right lines, by the 1st Law. But if they do
agitate one another with some certain forces, and those forces are as the
diameters of the correspondent particles inversely and the squares of the
velocities directly, then, because the particles are in like situations, and
their forces are proportional, the whole forces with which correspondent
particles are agitated, and which are compounded of each of the agitating
forces (by Corol. 2 of the Laws), will have like directions, and have the
same effect as if they respected centres placed alike among the particles;
and those whole forces will be to each other as the several forces which
compose them, that is, as the diameters of the correspondent particles in¬
versely, and the squares of the velocities directly : and therefore will caus»*

324

THE MATHEMATICAL PRINCIPLES

[Book lL

correspondent particles to continue to describe like figures. These things
will be so (by Gor. 1 and S, Prop. IV., Book 1), if those centres are at rest
but if they are moved, yet by reason of the similitude of the translation^
their situations among the particles of the system will remain similar, so
that the changes introduced into the figures described by the particles will
Btill be similar. So that the motions of correspondent and similar par¬
ticles will continue similar till their first meeting with each other; and
thence will arise similar collisions, and similar reflexions; which will again
beget similar motions of the particles among themselves (by what was just
now shown), till they mutually fall upon one another again, and so on ad
infinitum.

Cor. 1 . Hence if any two bodies, which are similar and in like situations
to the correspondent particles of the systems, begin to move amongst them
in like manner and in proportional times, and their magnitudes and densi¬
ties be to each other as the magnitudes and densities of the corresponding
particles, these bodies will continue to be moved in like manner and in
proportional times; for the case of the greater parts of both systems and of
the particles is the very same.

Cor. 2. And if all the similar and similarly situated parts of both sys¬
tems be at rest among themselves; and two of them, which are greater than
the rest, and mutually correspondent in both systems, begin to move in
lines alike posited, with any similar motion whatsoever, they will excite
similar motions in the rest of the parts of the systems, and will continue
to move among those parts in like manner and in proportional times ; and
will therefore describe spaces proportional to their diameters.

PROPOSITION XXXIII. THEOREM XXVII.

The same things hiring supposed, I say, that the greater parts of the
systems are resisted in a ratio compounded of the duplicate ratio of
their velocities , and the duplicate ratio of their diameters, and the sim¬
ple ratio of the density of the parts of the systems.

For the resistance arises partly from the centripetal or centrifugal, forces
with which the particles of the system mutually act on each other, partly
from the collisions and reflexions of the particles and the greater parts.
The resistances of the first kind are to each other as the whole motive
forces from which they arise, that is, as the whole accelerative forces and
the quantities of matter in corresponding parts; that is (by the sup¬
position), as the squares of the velocities directly, and the distances of the
corresponding particles inversely, and the quantities of matter in the cor¬
respondent parts directly : and therefore since the distances of the parti¬
cles in one system are to the correspondent distances of the particles of the
flher as the diameter of one particle or part in *he former system to the

OF NATURAL PHILOSOPHY.

o25

Sec. VII.]

diameter of the correspondent particle or part in the other, and since the
quantities of matter are as the densities of the parts and the cubes of the
diameters; the resistances arc to each other as the squares of the velocities
and the squares of the diameters and the densities of the parts of the sys¬
tems. Q.E.D. The resistances of the latter sort are as the number of
correspondent reflexions and the forces of those reflexions conjunctly; but
the number of the reflexions are to each other as the velocities of the cor¬
responding parts directly and the spaces between their reflexions inversely.
And the forces of the reflexions are as the velocities and the magnitudes
and the densities of the corresponding parts conjunctly; that is, as the ve¬
locities and the cubes of the diameters and the densities of the parts. And,
joining all these ratios, the resistances of the corresponding parts are to
each other as the squares of the velocities and the squares of the diameters
and the densities of the parts conjunctly. Q,.E.D.

Cor. 1. Therefore if those systems are two elastic fluids, like our air,
and their parts are at rest among themselves; and two similar bodies pro¬
portional in magnitude and density to the parts of the fluids, and similarly
situated among those parts, be any how projected in the direction of lines
similarly posited; and the accelerative forces with which the particles of
the fluids mutually act upon each other are as the diameters of the bodies
projected inversely and the squares of their velocities directly; those bodies
will excite similar motions in the fluids in proportional times, and will de¬
scribe similar spaces and proportional to their diameters.

Cor. 2. Therefore in the same fluid a projected body that moves swiftly
meets with a resistance that is, in the duplicate ratio of its velocity, nearly.
For if the forces with which distant particles act mutually upon one
another should be augmented in the duplicate ratio of the velocity, the
projected body would be resisted in the same duplicate ratio accurately;
and therefore in a medium, whose parts when at a distance do not act mu¬
tually with any force on one another, the resistance is in the duplicate ra¬
tio of the velocity accurately. Let there be, therefore, three mediums A,
B, C, consisting of similar and equal parts regularly disposed at equal
distances. Let the parts of the mediums A and B recede from each other
with forces that are among themselves as T and V; and let the parts of
the medium C be entirely destitute of any such forces. And if four equal
bodies D, E, F, G, move in these mediums, the two first D and E in the
two first A and B, and the other two F and G in the third C; and if the
velocity of the body D be to the velocity of the body E, and the velocity
of the body F to the velocity of the body G, in the subduplicate ratio of
the force T to the force V; the resistance of the body D to the resistance
of the body E, and the resistance of the body F to the resistance of the
body G, will be in the duplicate ratio of the velocities ; and therefore the
resistance of the body D will be to the resistance of the body F as the re-

326

THE MATHEMATICAL PRINCIPLES

[Book II

sistance of the body E to the resistance of the body G. Let the bodies 1)
and F be equally swift, as also the bodies E and G; and, augmenting the
velocities of thejbodies D and F in any ratio, and diminishing the forces
of the particles of the medium B in the duplicate of the same ratio, the
medium B will approach to the form and condition of the medium C at
pleasure; and therefore the resistances of the equal and equally swift
bodies E and G in these mediums will perpetually approach to equality
so that their difference will at last become less than any given. There¬
fore since the resistances of the bodies 1) and F are to each other as the
resistances of the bodies E and G, those will also in like manner approach
to the ratio of equality. Therefore the bodies D and F, when they move
with very great swiftness, meet with resistances very nearly equal; and
therefore since the resistance of the body F is in a duplicate ratio of the
velocity, the resistance of the body D will be nearly in the same ratio.

Cor. 3. The resistance of a body moving very swift in an elastic fluid
is almost the same as if the parts of the fluid were destitute of their cen¬
trifugal forces, and did not fly from each other; if so be that the elasti¬
city of the fluid arise from the centrifugal forces of the particles, and the
velocity be so great as not to allow the particles time enough to act.

Cor. 4. Therefore, since the resistances of similar and equally swift
bodies, in a medium whose distant parts do not fly from each other, are as
the squares of the diameters, the resistances made to bodies moving with
very great and equal velocities in an elastic fluid will be as the squares of
the diameters, nearly.

Cor. 5. And since similar, equal, and equally swift bodies, moving
through mediums of the same density, whose particles do not fly from each
other mutually, will strike against an equal quantity of matter in equal
times, whether the particles of which the medium consists be more and
smaller, or fewer and greater, and therefore impress on that matter an equal
quantity of motion, and in return (by the 3d Law of Motion) suffer an
equal re-action from the same, that is, are equally resisted; it is manifest,
also, that in elastic fluids of the same density, when the bodies move with
extreme swiftness, their resistances are nearly equal, whether the fluids
consist of gross parts, or of parts ever so subtile. For the resistance of
projectiles moving with exceedingly great celerities is not much diminished
by the subtilty of the medium.

Cor. G. All these things are so in fluids whose elastic force takes its rise
from the centrifugal forces of the particles. But if that force arise from
some other cause, as from the expansion of the particles after the manner
of wool, or the boughs of trees, or any other cause, by which the particles
are hindered from moving freely among themselves, the resistance, by
reason of the lesser fluidity of the medium, will be greater than in the
Corollaries above.

OK NATURAL PHILOSOPHY.

32?

Sec. YII.J

PROPOSITION XXXIY. THEOREM XXYII1.

If in a rare medium , consisting of equal particles freely disposed at
equal distances from each other , a globe and a cylinder described on
equal diameters move with equal velocities in the direction of the axis
of the cylinder , the resistance of the globe loill be but half so great as
that of the cylinder.

For since the action of the medi¬
um upon the body is the same (by
Cor. 5 of the Laws) whether the body
move in a quiescent medium, or
whether the particles of the medium
impinge with the same velocity upon
the quiescent body, let us consider
the body as if it were quiescent, and
see with what force it would be im¬
pelled by the moving medium. Let, therefore, ABKl represent a spherical
body described from the centre C with the semi-diameter CA, and let the
particles of the medium impinge with a given velocity upon that spherical
body in the directions of right lines parallel to AC; and let FB be one of
those right lines. In FB take LB equal to the semi-diameter CB, and
draw BI) touching the sphere in B. Upon KC and BD let fall the per¬
pendiculars BE, LD; and the force with which a particle of the medium,
impinging on the globe obliquely in the direction FB, would strike the
globe in B, will be to the,force with which the same particle, meeting the
cylinder ONGQ, described about the globe with the axis ACI, would strike
it perpendicularly in b , as LD to LB, or BE to BC. Again; the efficacy
of this force to move the globe, according to the direction of its incidence
FB or AC, is to the efficacy of the same to move the globe, according to
the direction of its determination, that is, in the direction of the right line
BC in which it impels the globe directly, as BE to BC. And, joining
these ratios, the efficacy of a particle, falling upon the globe obliquely in
the direction of the right line FB, to move the globe in the direction, of its
incidence, is to the efficacy of the same particle falling in the same line
perpendicularly on the cylinder, to move it in the same direction, as BE 2
to BC 2 . Therefore if in £E, which is perpendicular to the circular base of
the cylinder NAO, and equal to the radius AC, we take £H equal to
BE a

; then bH will be to 6E as the effect of the particle upon the globe tc

the effect of the particle upon the cylinder. And therefore the solid which
is formed by all the right lines 6H will be to the solid formed by all the
right lines bJZ as the effect of all the particles upon the globe to the effect
of all the particles upon the cylinder. But the former of these solids is a

G k jsr

32S

THE MATHEMATICAL PRINCIPLES

[Book 11.

paraboloid whose vertex is C, its axis CA, and latus rectum CA, and the
latter solid is a cylinder circumscribing the paraboloid; and it is know'r
that a paraboloid is half its circumscribed cylinder. Therefore the whole
force of the medium upon the globe is half of the entire force of the same
upon the cylinder. And therefore if the particles of the medium are at
rest, and the cylinder and globe move with equal velocities, the resistance
of the globe will be half the resistance of the cylinder. Q.E.D.

SCHOLIUM.

By the same method other figures may be compared together as to their
resistance; and those may be found which are most apt to continue their
motions in resisting mediums. As if upon the circular base CEBH from
the centre O, with the radius OC, and the altitude OD, one would construct
a frustum CBGF of a cone, which should meet with less resistance than
any other frustum constructed with the same base and altitude, and going
forwards towards D in the direction of its axis: bisect the altitude OD in
Li, and produce OQ, to S, so that QS may be equal to QC, and S will be
the vertex of the cone whose frustum is sought.

Whence, by the bye, since the angle CSB is always acute, it follows, that,
if the solid ADBE be generated by the convolution of an elliptical or oval
figure ADBE about its axis AB, and the generating figure be touched by
three right lines FG, GH, HI, in the points F, B, and I, so that GH shall
be perpendicular to the axis in the point of contact B, and FG, HI may be
inclined to GH in the angles FGB, BHI of 135 degrees: the solid arising
from the convolution of the figure ADFGH1E about the same axis AB
will be less resisted than the former solid; if so be that both move forward
in the direction of their axis AB, and that the extremity B of each go
foremost. Which Proposition I conceive may be of use in the building of
ships.

If the figure DNFG be such a curve, that if, from any point thereof, as
N, the perpendicular NM be let fall on the axis AB, and from the given
point G there be drawn the right line GR parallel to a right line touching
the figure in N, and cutting the axis produced in R, MN becomes to GR
as GR 3 to dBR X GB 2 , the solid described by the revolution of this figure

OF NATURAL PHILOSOPHY.

32S

Sec. V 11.1

about its axis AB, moving in the before-mentioned rare medium from A
towards B, will be less resisted than any other circular solid whatsoever,
described of the same length and breadth.

PROPOSITION XXXY. PROBLEM VII.

If a rare medium consist of very small quiescent particles of equal mag•
nitudes , and freely disposed at equal distances from one another: to
find the resistance of a globe moving uniformly forward in this
medium.

Case 1. Let a cylinder described with the same diameter and altitude be
conceived to go forward with the same velocity in the direction of its axis
through the same medium; and let us suppose that the particles of the
medium, on which the globe or cylinder falls, fly back with as great a force
of reflexion as possible. Then since the resistance of the globe (by the last
Proposition) is but half the resistance of the cylinder, and since the globe
is to the cylinder as 2 to 3, and since the cylinder by falling perpendicu¬
larly on the particles, and reflecting them with the utmost force, commu¬
nicates to them a velocity double to its own; it follows that the cylinder*
in moving forward uniformly half the length of its axis, will communicate
a motion to the particles which is to the whole motion of the cylinder as
the density of the medium to the density of the cylinder; and that the
globe, in the time it describes one length of its diameter in moving uni¬
formly forward, will communicate the same motion to the particles; and
in the time that it describes two thirds of its diameter, will communicate
a motion to the particles which is to the whole motion of the globe as the
density of the medium to the density of the globe. And therefore the
globe meets with a resistance, which is to the force by which its whole mo¬
tion may be either taken away or generated in the time in which it de¬
scribes two thirds of its diameter moving uniformly forward, as the den¬
sity of the medium to the density of the globe.

Case 2. Let us suppose that the particles of the medium incident on
the globe or cylinder are not reflected; and then the cylinder falling per¬
pendicularly on the particles will communicate its own simple velocity to
them, and therefore meets a resistance but half so great as in the former
case, and the globe also meets with a resistance but half so great.

Case 3. Let us suppose the particles of the medium to fly back from
the globe with a force which is neither the greatest, nor yet none at all, but
with a certain mean force: then the resistance of the globe will be in the
same mean ratio between the resistance in the first case and the resistance
in the second. Q.E.I.

Cor. 1. Hence if the globe and the particles are infinitely hard, and
destitute of all elastic force, and therefore of all force of reflexion; th«
resistance of the globe will be to the force by which its whole motion may

330

THE MATHEMATICAL PRINCIPLES

[Book I)

be destroyed or generated, in the time that the globe describes four third
parts of its diameter, as the density of the medium to the density of the
globe.

Cor. 2. The resistance of the globe, cceteris paribus, is in the duplicate
ratio of the velocity.

Cor. 3. The resistance of the globe, cccterisparibus , is in the duplicate
ratio of the diameter.

Cor. 4. The resistance of the globe is, cceteris paribus, as the density of
the medium.

Cor. 5. The resistance of the globe is in a ratio compounded of the du¬
plicate ratio of the velocity, and the duplicate ratio of the diameter, and
the ratio of the density of the medium.

Cor. 6. The motion of the globe and its re¬
sistance may be thus expounded Let AB be the
time in which the globe may, by its resistance
uniformly continued, lose its whole motion.
Erect AD, BC perpendicular to AB. Let BC be
that whole motion, and through the point C, the
asymptotes being AD, AB, describe the hyperbola
CF. Produce AB to any point E. Erect the perpendicular EF meeting
the hyperbola in F. Complete the parallelogram CBEG, and draw AF
meeting BC in H. Then if the globe in any time BE, witii its first mo¬
tion BC uniformly continued, describes in a non-resisting medium the space
CBEG expounded by the area of the parallelogram, the same in a resisting
medium will describe the space CBEF expounded by the area of the hy¬
perbola ; and its motion at the end of that time will be expounded by EF,
the ordinate of the hyperbola, there being lost of its motion the part FG.
And its resistance at the end of the same time will be expounded by the
length BH. there being lost of its resistance the part CH. All these things
appear by Cor. 1 and 3, Prop. V., Book II.

Cor. 7. Hence if the globe in the time T by the resistance R uniformly
continued lose its whole motion M, the same globe in the time t in a
resisting medium, wherein the resistance R decreases in a duplicate

/M

ratio of the velocity, will lose out of its motion M the part , ; ■ —/ the

T + t

TM

de¬

part pp—py remaining; and will describe a space which is to the space

scribed in the same time t } with the uniform motion M, as the logarithm of
T -f t

the number —multiplied by the number 2,3025S5092994 is to the
t

number —, because the hyperbolic area BCFE is to the rectangle BCGE
in that proportion.

Sec. VII.]

OF NATURAL PHILOSOPHY*.

331

SCHOLIUM.

1 have exhibited in this Proposition the resistance and retardation of
Bpherical projectiles in mediums that are not continued, and shewn that
this resistance is to the force by which the whole motion of the globe may be
destroyed or produced in the time in which the globe can describe two thirds
of its diameter, with a velocity uniformly continued, as the density of the
medium to the density of the globe, if so be the globe and the particles of
the medium be perfectly elastic, and are endued with the utmost force of
reflexion; and that this force, where the globe and particles of the medium
are infinitely hard and void of any reflecting force, is diminished one half.
But in continued mediums, as water, hot oil, and quicksilver, the globe as
it passes through them does not immediately strike against all the parti¬
cles of the fluid that generate the resistance made to it, but presses only
the particles that lie next to it, which press the particles beyond, which
press other particles, and so on ; and in these mediums the resistance is di¬
minished one other half. A globe in these extremely fluid mediums meets
with a resistance that is to the force by which its whole motion may be
destroyed or generated in the time wherein it can describe, with that mo¬
tion uniformly continued, eight third parts of its diameter, as the density
of the medium to the density of the globe. This I shall endeavour to shew
in what follows.

PROPOSITION XXXVI. PROBLEM VIII.

To define the motion of water running out of a cylindrical vessel through
a hole made at the bottom.

Let AC DB be a cylindrical vessel, AB the mouth
of it, CD the bottom p irallel to the horizon, EF a
circular hole in the middle of the bottom, G the
centre of the hole, and GH the axis of the cylin¬
der perpendicular to the horizon. And suppose a
cylinder of ice APQ,B to be of the same breadth
with the cavity of the vessel, and to have the same
axis, and to descend perpetually with an uniform
motion, and that its parts, as soon as they touch the
superficies AB, dissolve into water, and flow
wn by their weight into the vessel, and in their
fall compose the cataract or column of water
ABNFEM, passing through the hole EF, and filling up the same exactly.
Let the uniform velocity of the descending ice and of the contiguous water
in the circle AB be that which the water would acquire by falling through
the space IH ; and let IH and HG lie in the same right line; and through

332

THE MATHEMATICAL PRINCIPLES

[Book II

the point I let there be drawn the right line KL parallel to the horizon,
and meeting the ice on both the sides thereof in K and L. Then the ve¬
locity of the water running out at the hole EF will be the same that it
would acquire by falling from I through the space IG. Therefore, by
Galileo’s Theorems, IG will be to IH in the duplicate ratio of the velo¬
city of the water that runs out at the hole to the velocity of the w r ater in
the circle AB, that is, in the duplicate ratio of the circle AB to the circle
EF; those circles being reciprocally as the velocities of the water which
in the same time and in equal quantities passes severally through each of
them, and completely fills them both. We are now considering the velo¬
city with which the water tends to the plane of the horizon. But the mo¬
tion parallel to the same, by which the parts of the falling w r ater approach to
each other, is not here taken notice of; since it is neither produced by
gravity, nor at all changes the motion perpendicular to the horizon which the
gravity produces. We suppose, indeed, that the parts of the water cohere
a little, that by their cohesion they in ay in falling approach to each othei
with motions parallel to the horizon in order to form one single cataract,
and to prevent their being divided into several: but the motion parallel to
the horizon arising from this cohesion does not come under our present
consideration.

Case 1. Conceive now the whole cavity in the vessel, which encompasses
the falling water ABNFEM, to be full of ice, so that the water may pass
through the ice as through a funnel. Then if the water pass very near to
the ice only, wfithout touching it; or, which is the same thing, if by rea¬
son of the perfect smoothness of the surface of the ice, the water, though
touching it, glides over it with the utmost freedom, and without the least
resistance ; the water will run through the hole EF with the same velocity
as before, and the whole weight of the column of water ABNFEM will be
all taken up as before in forcing out the water, and the bottom of the vessel
will sustain the weight of the ice encompassing that column.

Let now the ice in the vessel dissolve into water ; yet will the efflux of
the water remain, as to its velocity, the same as before. It will not be
less, because the ice now dissolved will endeavour to descend; it will not
be greater, because the ice, now' become water, cannot descend without hin¬
dering the descent of other water equal to its own descent. The same force
ought always to generate the same velocity in the effluent water.

But the hole at the bottom of the vessel, by reason of the oblique mo¬
tions of the particles of the effluent water, must be a little greater than before*
For now the particles of the water do not all of them pass through the
hole perpendicularly, but, flowing down on all parts from the sides of the
vessel, and converging towards the hole, pass through it with oblique mo¬
tions : ar.d in tending downwards meet in a stream whose diameter is a little
smaller below the hole than at the hole itself: its diameter being to the

Sec. VII.!

OF NATURAL PHILOSOPHY.

333

diameter of the hole as 5 to 6, or as 5£ to 6J, very nearly, if I took the
measures of those diameters right. I procured a very thin flat plate, hav¬
ing a hole pierced in the middle, the diameter of the circular hole being
f parts of an inch. And that the stream of running waters might not be
accelerated in falling, and by that acceleration become narrower, I fixed
this plate not to the bottom, but to the side of the vessel, so as to make the
water go out in the direction of a line parallel to the horizon. Then, when
the vessel was full of water, I opened the hole to let it run out; and the
diameter of the stream, measured with great accuracy at the distance of
about half an inch from the hole, was || of an inch. Therefore the di¬
ameter .of this circular hole was to the diameter of the stream very nearly
as 25 to 21. So that the water in passing through the hole converges on
all sides, and, after it has run out of the vessel, becomes smaller by converg¬
ing in that manner, and by becoming smaller is accelerated till it comes to
the distance of half an inch from the hole, and at that distance flows in a
smaller stream and with greater celerity than in the hole itself, and this
in the ratio of 25 X 25 to 21 X 21, or 17 to 12, very nearly; that is, in
about the subduplicate ratio of 2 to 1. Now it is certain from experiments,
that the quantity of water running out in a given time through a circular
hole made in the bottom of a vessel is equal to the quantity, which, flow¬
ing with the aforesaid velocity, would run out in the same time througli
another circular hole, whose diameter is to the diameter of the former as
21 to 25. And therefore that running water in passing through the
hole itself has a velocity downwards equal to that which a heavy body
would acquire in falling through half the height of the stagnant water in
the vessel, nearly. But, then, after it has run out, it is still accelerated by
converging, till it arrives at a distance from the hole that is nearly equal to
its diameter, and acquires a velocity greater than the other in about the
. subduplicate ratio of 2 to 1; which velocity a heavy body would nearly
acquire by falling through the whole height of the stagnant water in the
vessel.

Therefore in what follows let the diameter of
-,B the stream be represented by that lesser hole which
we called EF. And imagine another plane V YV
above the hole EF, and parallel to the plane there¬
of, to be placed at a distance equal to the diame¬
ter of the same hole, and to be pierced through
with a greater hole ST, of such a magnitude that
a stream which will exactly fill the lower hole EF
E ^ 3? x> may pass through it; the diameter of which hole
will therefore be to the diameter of the lower hole as 25 to 21, nearly. By
this means the water will run perpendicularly out at the lower hole; and
the quantity of the water running out will be, according to the magnitude

X H

\ °

JVl\

\n

/

Y... _'

It w

\z

[_

334

THE MATHEMATICAL PRINCIPLES

[Book II

of this last hole, the same, very nearly, which the solution of the Problem
requires. The space included between the two planes and the falling stream
may be considered as the bottom of the vessel. But, to make the solution
more simple and mathematical, it is better to take the lower plane alone
for the bottom of the vessel, and to suppose that the water which flowed
through the ice as through a funnel, and ran out of the vessel through the
hole EF made in the lower plane, preserves its motion continually, and that
the ice continues at rest. Therefore in what follows let ST be the diame¬
ter of a circular hole described from the centre Z, and let the stream run
out of the vessel through that hole, when the water in the vessel is all
fluid. And let EF be the diameter of the hole, which the stream, in fall¬
ing through, exactly fills up, whether the water runs out of the vessel by
that upper hole ST, or flows through the middle of the ice in the vessel,
as through a funnel. And let the diameter of the upper hole ST be to the
diameter of the lower EF as about 25 to 21, and let the perpendicular dis
tance between the pjanes of the holes be equal to the diameter of the lesser
hole EF. Then the velocity of the water downwards, in running out of
the vessel through the hole ST, will be in that hole the same that a body
may acquire by falling from half the height IZ; and the velocity of both
the falling streams will be in the hole EF, the same which a body would
acquire by falling from the whole height IG.

Case 2. If the hole EF be not in the middle of the bottom of the ves¬
sel, but in some other part thereof, the water will still run out with the
same velocity as before, if the magnitude of the hole be the same. For
though an heavy body takes a longer time in descending to the same depth,
by an oblique line, than by a perpendicular line, yet in both cases it acquires
in its descent the same velocity; as Galileo has demonstrated.

Case 3. The velocity of the water is the same when it runs out through
a hole in the side of the vessel. For if the hole be small, so that the in¬
terval between the superficies AB and KL may vanish as to sense, and the
stream of water horizontally issuing out may form a parabolic figure; from
the latus rectum of this parabola may be collected, that the velocity of the
effluent water is that which a body may acquire by falling the height IG
or HG of the stagnant water in the vessel. For, by making an experi¬
ment, I found that if the height of the stagnant water above the hole were
20 inches, and the height of the hole above a plane parallel to the horizon
were also 20 inches, a stream of water springing out from thence would
fall upon the plane, at the distance of 37 inches, very nearly, from a per¬
pendicular let fall upon that plane from the hole. For without resistance
the stream would have fallen upon the plane at the distance of 40 inches,
the latus rectum of the parabolic stream being 80 inches.

Case 4. If the effluent water tend upward, it will still i?sue forth with
the same velocity. For the small stream of water springing upward, as-

Sec. Vll.j

OF NATURAL PHILOSOPHY.

335

cends with a perpendicular motion to GH or GI, the height of the stagnant
water in the vessel; excepting in so far as its ascent is hindered a little by
the resistance of the air ; and therefore it springs out with the same ve¬
locity that it would acquire in falling from that height. Every particle of
the stagnant water is equally pressed on all sides (by Prop. XIX., Book II),
and, yielding to the pressure, tends always with an equal force, whether it
descends through the hole in the bottom of the vessel, or gushes out in an
horizontal direction through a hole in the side, or passes into a canal, and
springs up from thence through a little hole made in the upper part of the
canal. And it may not only be collected from reasoning, but is manifest
also from the well-known experiments just mentioned, that the velocity
with which the water runs out is the very same that is assigned in this
Proposition.

Case 5. The velocity of the effluent water is the same, whether the
figure of the hole be circular, or square, or triangular, or any other figure
equal to the circular; for the velocity of the effluent water does not depend
upon the figure of the hole, but arises from its depth below the plane

KL.

Case 6. If the lower part of the vessel ABI)C
B be immersed into stagnant water, and the height
of the stagnant water above the bottom of the ves¬
sel be GR, the velocity with which the water that
is in the vessel will run out at the hole EF into
the stagnant water will be the same which the
water would acquire by falling from the height
IR; for the weight of all the water in the vessel
1 that is below the superficies of the stagnant water
will be sustained in equilibrio by the weight of the stagnant water, and
therefore does not at all accelerate the motion of the descending water in
the vessel. This case will also appear by experiments, measuring the times
in which the water will run out.

Cor. 1. Hence if CA the depth of the water be produced to X, so that
AK may be to CK in the duplicate ratio of the area of a hole made in any
part of the bottom to the area of the circle AB, the velocity of the effluent
water will be equal to the velocity which the water would acquire by falling
from the height KC.

Cor. 2. And the force with which the whole motion of the effluent watei
may be* generated is equal to the weight of a cylindric column of water,
whose base is the hole EF, and its altitude 2GI or 2CK. For the effluent
water, in the time it becomes equal to this column, may acquire, by falling
by its own weight from the height GI, a velocity equal to that with which
it runs out.

Cor. 3. The weight of all the water in the vessel ABDC is to that part

336

THE MATHEMATICAL PRINCIPLES

[Book II

of the weight which is employed in forcing out the water as the sum of
the circles AB and EF to twice the circle EF. For let 10 be a mean pro¬
portional between IH and IG, and the water running out at the hole EF
will, in the time that a drop falling from I would describe the altitude IG,
become equal to a cylinder whose base is the circle EF and its altitude
2IG, that is, to a cylinder whose base is the circle AB, and whose altitude
is 210. For the circle EF is to the circle AB in the subduplicate ratio cf
the altitude IH to the altitude IG; that is, in the simple ratio of the mean
proportional 10 to the altitude IG. Moreover, in the time that a drop
falling from I can describe the altitude IH, the water that runs out will
have become equal to a cylinder whose base is the circle AB, and its alti¬
tude 2IH; and in the time that a drop falling from I through H to G de¬
scribes HG, the difference of the altitudes, the effluent water, that is, the
water contained within the solid ABNFEM, will be equal to the difference
of the cylinders, that is, to a cylinder whose base is AB, and its altitude
2H0. And therefore all the water contained in the vessel ABDC is to the
whole falling water contained in the said solid ABNFEM as HG to 2H0,
that is, as HO -f OG to 2H0, or IH + I() to 2IH. But the weight of all
the water in the solid ABNFEM is employed in forcing out the water ;
and therefore the weight of all the water in the vessel is to that part of
the weight that is employed in forcing out the water as IH + 10 to 2IH,
and therefore as the sum of the circles EF and AB to twice the circle
EF.

Cor. 4. And hence the weight of all the water in the vessel ABDC is
to the other part of the weight which is sustained by the bottom of the
vessel as the sum of the circles AB and EF to the difference of the same
circles.

Cor. 5. And that part of the weight which the bottom of the vessel sus¬
tains is to the other part of the weight employed in forcing out the water
as the difference of the circles AB and EF to twice the lesser circle EF, or
as the area of the bottom to twice the hole.

Cor. 6. That part of the weight which presses upon the bottom is to
the whole weight of the water perpendicularly incumbent thereon as the
circle AB to the sum of the circles AB and EF, or as the circle AB to thf
excess of twice the circle AB above the area of the bottom. For that part
of the weight which presses upon the bottom is to the weight of the whole
water in the vessel as the difference of the circles AB and EF to .the sum
of the same circles (by Cor. 4); and the weight of the whole water in the
vessel is to the weight of the whole water perpendicularly incumbent on
the bottom as the circle AB to the difference of the circles AB and EF.
Therefore, ex cequo perturbate, that part of the weight which presses upon
the bottom is to the weight of the whole water perpendicularly incumbent *

OF NATURAL PHILOSOPHY.

337

Sec* YU.!

K

thereon as the circle AB to the sum of the circles AB and EF. or the ex¬
cess of twice the circle AB above the bottom.

Cor. 7. If in the middle of the hole EF there be placed the little circle
PQ described about the centre G, and parallel to the horizon, the weight
of water which that little circle sustains is greater than the weight of a
third part of a cylinder of water whose base is that little circle and its
height GH. For let ABNFEM be the cataract or column of falling water
whose axis is GH, as above, and let all the wa¬
ter, whose fluidity is not requisite for the ready
and quick descent of the water, be supposed to
be congealed, as well round about the cataract,
as above the little circle. And let PHQ be the
column of water congealed above the little cir¬
cle, whose vertex is H, and its altitude GH.

And suppose this cataract to fall with its whole
weight downwards, and not in the least to lie
against or to press PHQ, but to glide freely by
it without any friction, unless, perhaps, just at
the very vertex of the ice, where the cataract at the beginning of its fall
may tend to a concave figure. And as the congealed water AMEC, BNFD,
lying round the cataract, is convex in its internal superficies AME, BNF,
towards the falling cataract, so this column PHQ will be convex towards
the cataract also, and will therefore be greater than a cone whose base is

that little circle PQ and its altitude GH; that

is, greater

than a third

part of a cylinder described with the same base and altitude. Now that
little circle sustains the weight of this column, that is, a weight greater
than the weight of the cone, or a third part of the cylinder.

Cor. S. The weight of water which the circle PQ ; when very small, sus¬
tains, seems to be less than the weight of two thirds of a cylinder of water
whose base is that little circle, and its altitude HG. For, things standing
as above supposed, imagine the half of a spheroid described whose base is
that little circle, and its semi* axis or altitude HG. This figure will be
equal to two thirds of that cylinder, and will comprehend within it the
column of congealed water PHQ, the weight of which is sustained by that
little circle. For though the motion of the water tends directly down¬
wards, the external superficies of that column must yet meet the base PQ
in an angle somewhat acute, because the water in its fall is perpetually ac¬
celerated, and by reason of that acceleration become narrower. Therefore,
since that angle is less than a right one, this column in the lower parts
thereof will lie within the hemi-spheroid. In the upper parts also it will be
acute or pointed; because to make it otherwise, the horizontal motion of
the water must be at the vertex infinitely more swift than its motion to¬
wards the horizon. And the less this circle PQ i3, the more acute will

22

338

THE MATHEMATICAL PRINCIPLES

[Book II

the vertex of this column be ; and the circle being diminished in infinitum
the angle PHQ will be diminished in infinitum and therefore the co¬
lumn will lie within the hemi-spheroid. Therefore that column is less than
that hemi-spheroid, or than two-third parts of the cylinder whose base is
that little circle, and its altitude GH. Now the little circle sustains a
force of water equal to the weight of this column, the weight of the ambient
water being employed in causing its efflux out at the hole.

Cor. 9. The weight of water which the little circle PQ sustains, when
it is very small, is very nearly equal to the weight of a cylinder of water
whose base is that little circle, and its altitude |GH; for this weight is an
arithmetical mean between the weights of the cone and the hemi-spheroid
above mentioned. But if that little circle be not very small, but on the
contrary increased till it be equal to the hole EF, it will sustain the weight
of all the water lying perpendicularly above it, that is, the weight of a
cylinder of water whose base is that little circle, and its altitude GH.

Cor. 10. And (as far as I can judge) the weight which this little circle
sustains is always to the weight of a cylinder of water whose base is that
little circle, and its altitude |G1I, as EF 2 to EF 2 — iPQ 2 , or as the cir¬
cle EF to the excess of this circle above half the little circle PQ, very
nearly.

LEMMA IV.

If a cylinder move uniformly forward in. the direction of its length, the
resistance made thereto is not at all changed by augmenting or di¬
minishing that length ; and. is therefore the same with the resistance
of a circle, described with the same diameter , and moving forward
with the same velocity in the direction of a right line ■perpendicular to
its plane.

For the sides are not at all opposed to the motion ; and a cylinder be¬
comes a circle when its length is diminished in infinitum.

PROPOSITION XXXVII. THEOREM XXIX.

If a cylinder move uninformly forward in a compressed, infinite, and
non-elastic fluid, in the direction of its length, the resistance arising
from the magnitude of its transverse section is to the force by which
its whole motion may be destroyed or generated, in the time that it
moves four times its length, as the density of the medium to the den¬
sity of the cylinder, nearly.

For let the vessel ABDC touch the surface of stagnant water witli its
bottom CD, and let the water run out of this vessel into the stagnant wa¬
ter through the cylindric canal EFTS perpendicular to the horizon ; and
let the little circle PQ be placed parallel to the horizon any where in the

OF NATURAL PHILOSOPHY.

339

Sec. VII.]

middle of the canal; and produce CA to K, so
that AK may be to CK in the duplicate of the
ratio, which the excess of the orifice of the canal
EF above the little circle PQ bears to the cir¬
cle AB. Then it is manifest (by Case 5, Case
6, and Cor. 1, Prop. XXXVi) that the velocity
of the water passing through the an nular space
between the little circle and the sides of the ves¬
sel will be the very same which the water would
acquire by falling, and in its fall describing the
altitude KC or IG.

And (by Cor. 10, Prop. XXXVI) if the breadth of the vessel be infinite,
so that the lineola HI may vanish, and the altitudes IG, HG become equal;
the force of the water that flows down and presses upon the circle will be
to the weight of a cylinder whose base is that little circle, and the altitude
•ilG, as EF 2 to EF 2 — |PQ 2 , very nearly. For the force of the water
flowing downward uniformly through the whole canal will be the same
upon the little circle PQ, in whatsoever part of the canal it be placed.

Let now the orifices of the canal EF, ST be closed, and let the little
circle ascend in the fluid compressed on every side, and by its ascent let it
oblige the water that lies’ above it to descend through the annular space
between the little circle and the sides of the canal. Then will the velocity
tf the ascending little circle be to the velocity of the descending water as
the difference of the circles EF and PQ is to the circle PQ; and the ve¬
locity of the ascending little circle will be to the sum of the velocities, that
is, to the relative velocity of the descending water with which it passes by
the little circle in its ascent, as the difference of the circles EF and PQ to
the circle EF, or as EF 2 — PQ 2 to EF 2 . Let that relative velocity be
equal to the velocity with which it was shewn above that the water would
pass through the annular space, if the circle were to remain unmoved, that
is, to,the velocity which the water would acquire by falling, and in its fall
describing the altitude IG ; and the force of the water upon the ascending
circle will be the same as before (by Cor. 5, of the Laws of Motion); that
is, the resistance of the ascending little circle will be to the weight of a
cylinder of water whose base is that little circle, and its altitude |IG, as
EF 2 to EF 2 — iPQ 2 j nearly. But the velocity of the little circle will
be to the velocity which the water acquires by falling, and in its fall de¬
scribing the altitude IG, as EF 2 — PQ 2 to EF 2 .

Let the breadth of the canal be increased in infinitum ; and the ratios
between EF 2 — PQ 2 and EF 2 , and between EF 2 and EF 2 — £PQ 2 ,
will become at last ratios of equality. And therefore the velocity of the
little circle will now be the same which the water would acquire in falling,
and in its fall describing the altitude IG: and the resistance will become

K.I L

A

Hj

B

C

gI

D

E

p h-9

s

. J

T

340

THE MATHEMATICAL PRINCIPI ES

[Book IT.

&}ual to the weight of a cylinder whose base is that little circle, and its
altitude half the altitude IG, from which the cylinder must fall to acquire
the velocity of the ascending circle; and with this velocity the cylinder in
the time of its fall will describe four times its length. But the resistance
of the cylinder moving forward with this velocity in the direction of its
length is the same with the resistance of the little circle (by Lem. IV), and
is therefore nearly equal to the force by which its motion may be generated
while it describes four times its length.

If the length of the cylinder be augmented or diminished, its motion,
and the time in which it describes four times its length, will be augmented
or diminished in the same ratio, and therefore the force by which the mo¬
tion so increased or diminished, may be destroyed or generated, will con¬
tinue the same; because the time is increased or diminished in the same
proportion; and therefore that force remains still equal to the resistance
of the cylinder, because (by Lem. IV) that resistance will also remain the
same.

If the density of the cylinder be augmented or diminished, its motion,
and the force by which its motion may be generated or destroyed in the
same time, will be augmented or diminished in the same ratio. Therefore
the resistance of any cylinder whatsoever will be to the force by which its
whole motion may be generated or destroyed, in the time during which it
moves four times its length, as the density of the medium to the density of
the cylinder, nearly. Q.E.D.

A fluid must be compressed to become continued; it must be continued
and non-elastic, that all the pressure arising from its compression may be
propagated in an instant; and so, acting equally upon all parts of the body
moved, may produce no change of the resistance. The pressure arising
from the motion of the body is spent in generating a motion in the parts
of the fluid, and this creates the resistance. But the pressure arising from
the compression of the fluid, be it ever so forcible, if it be propagated in an
instant, generates no motion in the parts of a continued fluid, produces no
change at all of motion therein; and therefore neither augments nor les¬
sens the resistance. This is certain, that the action of the fluid arising
from the compression cannot be stronger on the hinder parts of the body
moved than on its fore parts, and therefore cannot lessen the resistance de¬
scribed in this proposition. And if its propagation be infinitely swifter
than the motion of the body pressed, it will not be stronger on the fore
parts than on the hinder parts. But that action will be infinitely
swifter, and propagated in an instant, if the fluid be continued and non¬
elastic.

Cor. 1. The resistances, made to cylinders going uniformly forward in
the direction of their lengths through continued infinite mediums are in a

Sec. VII.] of natural philosophy. 341

ratio compounded of the duplicate ratio of the velocities and the duplicate
ratio of the diameters, and the ratio of the density of the mediums.

Cor. 2. If the breadth of the canal be not infinitely increased but the
cylinder go forward in the direction of its length through an included
quiescent medium, its axis all the while coinciding with the axis of the
canal, its resistance will be to the force by which its whole motion, in the

time in which it describes four times its length, *.I.L

may be generated or destroyed, in a ratio com¬
pounded of the ratio of EF 2 to EF 2 —
once, and the ratio of EF 2 to EF 2 — PQ, 2
twice, and the ratio of the density of the medium
to the density of the cylinder.

Cor. 3. The same thing supposed, and that a
length L is to the quadruple of the length of
the cylinder in a ratio compounded of the ratio
EF 2 — |PQ, 2 to EF 2 once, and the ratio of
EF 2 —PQ, 2 to EF 2 twice; the resistance of
the cylinder will be to the force by which its whole motion, in the time
during which it describes the length L, may be destroyed or generated, as
the density of the medium to the density of the cylinder.

SCHOLIUM.

In this proposition we have investigated that resistance alone which
arises from the magnitude of the transverse section of the cylinder, neg¬
lecting that part of the same which may arise from the obliquity of the
motions. For as, in Case 1, of Prop. XXXVI., the obliquity of the mo¬
tions with which the parts of the water in the vessel converged on every
side to the hole EF hindered the efflux of the water through the hole, so,
in this Proposition, the obliquity of the motions, with which the parts of
the water, pressed by the antecedent extremity of the cylinder, yield to the
pressure, and diverge on all sides, retards their passage through the places
that lie round that antecedent extremity, toward the hinder parts of the
cylinder, and causes the fluid to be moved to a greater distance; which in¬
creases the resistance, and that in the same ratio almost in which it dimin¬
ished the efflux of the water out of the vessel, that is, in the duplicate ratio
of 25 to 21, nearly. And as, in Case 1, of that Proposition, we made the
parts of the water pass through the hole EF perpendicularly and in the
greatest plenty, by supposing all the water in the vessel lying round the
cataract to be frozen, and that part of the water whose motion was oblique,
and useless to remain without motion, so in this Proposition, that the
obliquity of the motions may be taken away, and the parts of the water
may give the freest passage to the cylinder, by yielding to it with the most
direct and quick motion possible, so that onlv so much resistance may re-

542

THE MATHEMATICAL PRINCIPLES

[Book II.

main as arises from the magnitude of the transverse section, and which is
incapable of diminution, unless by diminishing the diameter of the cylinder;
we must conceive those parts of the fluid whose motions are oblique and
useless, and produce resistance, to be at rest among themselves at both ex-
tremities of the cylinder, and there to cohere, and be joined to the cylinder.
Let ABCD be a rectangle, and let
AE and BE be two parabolic arcs,
described with the axis AB, and
with a latus rectum that is to the
space HG, which must be described ""—
by the cylinder in falling, in order
to acquire the velocity with which it moves, as HG to 4AB. Let CF and
DF be two other parabolic arcs described with the axis CD, and a latus
rectum quadruple of the former; and by the convolution of the figure
about the axis EF let there be generated a solid, whose middle part ABDC
is the cylinder we are here speaking of, and whose extreme parts ABE and
CDF contain the parts of the fluid at rest among themselves, and concreted
into two hard bodies, adhering to the cylinder at each end like a head and
tail. Then if this solid EACFDB move in the direction of the length of
its axis FE toward the parts beyond E, the resistance will be the same
which we have here determined in this Proposition, nearly; that is, it will
have the same ratio to the force with which the whole motion of the cyl¬
inder may be destroyed or generated, in the time that it is describing the
length 4x4.C with that motion uniformly continued, as the density of the
fluid has to the density of the cylinder, nearly. And (by Cor. 7, Prop.
XXXVI) the resistance must he to this force in the ratio of 2 to 3, at the
least.

LEMMA V.

If a cylinder, a sphere , and a spheroid, of equal breadths be placed suc¬
cessively in the middle of a cylindric canal, so that their axes may
coincide with the axis of the canal, these bodies will equally hinder the
passage of the water through the canal.

For the spaces lying between the sides of the canal, and the cylinder,
sphere, and spheroid, through which the water passes, are equal; and the
water will pass equally through equal spaces.

This is true, upon the supposition that all the water above the cylinder,
sphere, or spheroid, whose fluidity is not necessary to make the passage of
the water the quickest possible, is congealed, as was explained above in Cer
7, Prop. XXXVI.

Sec. VII.]

OF NATURAL PHILOSOPHY

343

LEMMA VI.

The same supposition, remaining , the fore-mentioned bodies are equally
acted on by the water flowing' through the canal.

This appears by Lem. V and the third Law. For the water and the
bodies act upon each other mutually and equally.

LEMMA VIL

If the water be at rest in the canal , and these bodies move with equal ve¬
locity and the contrary way through the canal, their resistances will
be equal among themselves.

This appears from the last Lemma, for the relative motions remain the
same among themselves.

SCHOLIUM.

The case is the same of all convex and round bodies, whose axes coincide
with the axis of the canal. Some difference may arise from a greater or
less friction; but in these Lemmata we suppose the bodies to be perfectly
smooth, and the medium to be void of all tenacity and friction; and that
those parts of the fluid which by their oblique and superfluous motions may
disturb, hinder, and retard the flux of the water through the canal, are at
nst among themselves; being fixed like water by frost, and adhering to
the fore and hinder parts of the bodies in the manner explained in the
Scholium of the last Proposition; for in what follows we consider the very
least resistance that round bodies described with the greatest given trans¬
verse sections can possibly meet with.

Bodies swimming upon fluids, when they move straight forward, cause
the fluid to ascend at their fore parts and subside at their hinder parts,
especially if they are of an obtuse figure; and thence they meet with a
little more resistance than if they were acut-e at the head and tail. And
bodies moving in elastic fluids, if they are obtuse behind and before, con¬
dense the fluid a little more at their fore parts, and relax the same at theii
hinder parts; and therefore meet also with a little more resistance than it
they were acute at the head and tail. But in these Lemmas and Proposi¬
tions we are not treating of elastic but non-elastic fluids; not of bodies
floating on the surface of the fluid, but deeply immersed therein. And
when the resistance of bodies in non-elastic fluids is once known, we may
then augment this resistance a little in elastic fluids, as our air; and in
the surfaces of stagnating fluids, as lakes and seas.

PROPOSITION XXXVIII. THEOREM XXX.

If a globe move uniformly forward in a compressed, infinite, and non•
elastic fluid , its resistance is to the force by which its whole motion

514

THE MATHEMATICAL PRINCIPLES

[Book II

may be destroyed or generated , in the time that it describes eight third
parts of its diameter , as the density of the fluid to the density of the
globe , very nearly.

For the globe is to its circumscribed cylinder as two to three; and there¬
fore the force which can destroy all the motion of the cylinder, while the
same cylinder is describing the length of four of its diameters, will destroy
all the motion of the globe, while the globe is describing two thirds of this
length, that is, eight third parts of its own diameter. Now the resistance
of the cylinder is to this force very nearly as the density of the fluid to the
density of the cylinder or globe (by Prop. XXXYI1), and the resistance of
the globe is equal to the resistance of the cylinder (by Lem. Y, VI, and
VII). Q.E.D.

Cor. 1. The resistances of globes in infinite compressed mediums are in
a ratio compounded of the duplicate ratio of the velocity, and the dupli¬
cate ratio of the diameter, and the ratio of the density of the mediums.

Cor. 2. The greatest velocity, with which a globe can descend by its
comparative weight through a resisting fluid, is the same which it may
acquire by falling with the same weight, and without any resistance, and'
in its fall describing a space that is, to four third parts of its diameter as
the density of the globe to the density of the fluid. For the globe in the
time of its fall, moving with the velocity acquired in falling, will describe
a space that will be to eight third parts of its diameter as the density of
the globe to the density of the fluid ; and the force of its weight which
generates this motion will be to the force that can generate the same mo¬
tion, in the time that the globe describes eight third parts of its diameter,
with the same velocity as the density of the fluid to the density of the
globe; and therefore (by this Proposition) the force of weight will be equal
to the force of resistance, and therefore cannot accelerate the globe.

Cor. 3. If there be given both the density of the globe and its velocity
at the beginning of the motion, and the density of the compressed quiescent
fluid in which the globe moves, there is given at any time both the velo¬
city of the globe and its resistance, and the space described by it (by Cor.
7, Prop. XXXV).

Cor. 4. A globe moving in a compressed quiescent fluid of the same
density with itself will lose half its motion before it can describe the length
of two of its diameters (by the same Cor. 7).

PROPOSITION XXXIX. THEOREM XXXI.

If a globe move uniformly forward through a fluid inclosed and com¬
pressed in a cylindric canal , its resistance is to the force by which its
whole motion may be generated or destroyed, in the time in which it
describes eight third parts of its diameter , in a ratio compounded of

OF NATURAL PHILOSOPHY.

345

Eo. VIT.J

the ratio of the orifice of the canal to the excess of that orifice above
half the greatest circle of the globe; and the duplicate ratio of the
orifice of the canal . to the excess of that orifice above the greatest circle
of the globe ; and the ratio of the density of the Jluid to the density of
the globe , nearly.

This appears by Cor. 2, Prop. XXXVII, and the demonstration pro¬
ceeds in the same manner as in the foregoing Proposition.

SCHOLIUM.

In the last two Propositions we suppose (as was done before in Lem. V)
that all the water which precedes the globe, and whose fluidity increases
the resistance of the same, is congealed. Now if that water becomes fluid,
it will somewhat increase the resistance. But in these Propositions that
increase is so small, that it may be neglected, because the convex superfi¬
cies of the globe produces the very same effect almost as the congelation
of the water.

PROPOSITION XL. PROBLEM IX.

To find by phenomena the resistance of a globe moving through a per¬
fectly fluid compressed medium.

Let A be the weight of the globe in vacuo , B its weight in the resisting
medium, D the diameter of the globe. F a space which is to f D as the den¬
sity of the globe to the density of the medium, that is, as A to A — B, G
the time in which the globe falling with the weight B without resistance
describes the space F, and H the velocity which the body acquires by that
fall. Then H will be the greatest velocity with which the globe can pos¬
sibly descend with the weight B in the resisting medium, by Cor. 2, Prop
XXXVIII; and the resistance which the globe meets with, when descend¬
ing with that velocity, will be equal to its weight B; and the resistance it
meets with in any other velocity will be to the weight B in the duplicate ra¬
tio of that velocity to the greatest velocity H, by Cor. 1, Prop. XXXVIII.

This is the resistance that arises from the inactivity of the matter of
the fluid. That resistance which arises from the elasticity, tenacity, and
friction of its parts, may be thus investigated.

Let the globe be let fall so that it may descend in the fluid by the weight
B ; and let P be the time of falling, and let that time be expressed in sec¬
onds, if the time G be given in seconds. Find the absolute number N

2P

agreeing to the logarithm 0,4342944819 and let L be the logarithm of
N + 1

the number —^—: and the velocity acquired in falling will he

*46

THE MATHEMATICAL PRINCIPLES

[Book 11

]\j_i 2PF

j- H, and the height described will be —-1.386294361 IF -f

4,6051701S6LF. If the fluid be of a sufficient depth, we may neglect the

2PF

term 4,605170186LF; and - — 1,3862943611F will be the altitude

described, nearly. These things appear by Prop. IX, Book II, and its Corol¬
laries, and are true upon this supposition, that the globe meet3 with no other
resistance but that which arises from the inactivity of matter. Now if it
really meet with any resistance of another kind, the descent will be slower,
and from the quantity of that retardation will be knowm the quantity of
this new resistance.

That the velocity and descent of a body falling in a fluid might more
easily be known, I have composed the following table ; the first column of
which denotes the times of descent; the second shews the velocities ac¬
quired in falling, the greatest velocity being 100000000: the third exhib¬
its the spaces described by falling in those times, 2F being the space which
the body describes in the time G with the greatest velocity ; and the fourth
gives the spaces described with the greatest velocity in the same times.

2P

The numbers in the fourth column are and by subducting the number

1,3862944 — 4,6051702L, are found the numbers in the third column;
and these numbers must be multiplied by the space F to obtain the spaces
described in falling. A fifth column is added to all these, containing the
spaces described in the same times by a body falling in vacuo with the
force of B its comparative weight.

The Times
P.

Velocities of the
body falling
in the fluid.

The spaces de
sciit ed in fall¬
ing in the fluid.

The spaces descri¬
bed with the
greatest motion.

The spaces de-J
scribed by fall¬
ing in vacuo.

0,0(>1G

99999|9

0.000001F

0,002F

0,000001 F

0,0lG

999967

0,000 IF

0.02F

r 0,000lF

0,lG

9966799

0,0099834F

0.2F

0,01F

0,2G

19737532

0.0397361F

0,4F

0.04F

0,3G

29131261

0.0886815F

0.6F

0.09F

0,4G

37994896

0,1559070F

0.8F

0,16F

0,5G

46211716

0,2402290F

1,0F

0,25F

0,6G

53704957

0,3402706F

1,2F

0.36F

0,7G

60436778

0.4545405F

1.4F

0,19F

0,8G

66403677

0,581507lF

1,6F

0,64F

0.9G

71629787

0.7196609F

1,SF

0.8 IF

1G

76159416

0.8675617F

2F

1 IF

2G

96402758

2,6500055F

4F

! 4F

3G

99505475

4.6186570F

6F

i 9F

4G

99932930

6,6143765F

8F

16F

5G

99990920

8.6137964F

10F

2 5F

6G

99998771

10,6137179F

12F

36F

7G

99999834

12.6137073F

14F

49F

8G

99999980

14.6137059F

16F

64F

9G

99999997

16!6137057F

18F

8lF

IOG

99999999f

18.6137056F

20F

100F

Sec. VII. |

OF NATURAL PHILOSOPHY.

347

SCHOLIUM.

In order to investigate the resistances of lluids from experiments, I pro¬
cured a square wooden vessel, whose length and breadth on the inside was
9 inches English measure, and its depth 9 feet \ ; this I filled with rain¬
water: and having provided globes made up of wax, and lead included
therein, I noted the times of the descents of these globes, the height through
which they descended being 112 inches. A solid cubic foot of English
measure contains 76 pounds troy weight of rain water; and a solid inch
contains if ounces troy weight, or 253>- grains; and a globe of water of
one inch in diameter contains 132,645 grains in air, or 132,8 grains in
vacuo; and any other globe will be as the excess of its weight in vacuo
above its weight in water.

Exper. 1. A globe whose weight was 156j grains in air, and 77 grains
in water, described the whole height of 112 inches in 4 seconds. And, upon
repeating the experiment, the globe spent again the very same time of 4
seconds in falling.

The weight of this globe in vacuo is I56if grains ; and the excess of
this weight above the weight of the globe in water is 7 9|f grains. Hence
the diameter of the globe appears to be 0,84224 parts of an inch. Then it
will be, as that excess to the weight of the globe in vacuo , so is the density
of the water to the density of the globe; and so is f parts of the diameter
of the globe (viz. 2,2459 7 inches) to the space 2F, which will be therefore
4,4256 inches. Now a globe falling in vacuo with its whole weight of
156^f grains in one second of time will describe 193| inches; and falling
in water in the same time with the weight of 77 grains without resistance,
will describe 95,219 inches*; and in the time G, which is to one second of
time in the subduplicate ratio of the space P, or of 2,2128 inches to 95,219
inches, will describe 2,2128 inches, and will acquire the greatest velocity H
with which it is capable of descending in water. Therefore the time G is
0",15244. And in this time G, with that greatest velocity H, the globe
will describe the space 2F, which is 4,4256 inches; and therefore in 4 sec¬
onds will describe a space of 116,1245inches. Subduct the space 1,3862944 F,
or 3,0676 inches, and there will remain a space of 113,0569 inches, which
the globe falling through water in a very wide vessel will describe in 4 sec¬
onds. But this space, by reason of the narrowness of the wooden vessel
before mentioned, ought to be diminished in a ratio compounded of the sub¬
duplicate ratio of the orifice of the vessel to the excess of this orifice above
half a great circle of the globe, and of the simple ratio of the same orifice
to its excess above a great circle of the globe, that is, in a ratio of 1 to
0,9914. This done, we have a space of 112,08 inches, which a globe fall¬
ing through the water in this wooden vessel in 4 seconds of time ought
nearly to describe by this theory; but it described 112 inches by the ex¬
periment.

348

THE MATHEMATICAL PRINCIPLES

[Book II

Exper. 2. Three equal globes, whose weights were severally 76} grains
in air, and 5~ w grains in water, were let fall successively; and every one
fell through the water in 15 seconds of time, describing in its fall a height
of 112 inches.

By computation, the weight of each globe in vacuo is 76 } 5 ¥ grains; the
excess of this weight above the weight in water is 71 grains J-J-; the diam¬
eter of the globe 0,81296 of an . inch ; | parts of this diameter 2,167St
inches; the space 2F is 2,3217 inches; the space which a globe of 5 T \
grains in weight would describe in one second without resistance, 12,80S
inches, and the time GO",301056* Therefore the globe, with the greatest
velocity it is capable of receiving from a weight of of- grains in its de¬
scent through water, will describe in the time 0",301056 the space of 2,3217
inches; and in 15 seconds the space 115,678 inches. Subduct the space
1,3862944F, or 1,609 inches, and there remains the space 114,069 inches,
which therefore the falling globe ought to describe in the same time, if the
vessel were very wide. But because our vessel was narrow, the space ought
to be diminished by about 0,895 of an inch. And so the space will remain
113,174 inches, which a globe falling in this vessel ought nearly to de¬
scribe in 15 seconds, by the theory. But by the experiment it described
112 inches. The difference is not sensible.

Exper. 3. Three equal globes, whose weights were severally 121 grains
in air, and 1 grain in water, were successively let fall; and they fell
through the water in the times 46", 47", and 50", describing a height ol
112 inches.

By the theory, these globes ought to have fallen in about 40". Now
whether their falling more slowly were occasioned from hence, that in slow
motions the resistance arising from the force of inactivity does really bear
a less proportion to the resistance arising from other causes; or whether
it is to be attributed to little bubbles that might chance to stick to the
globes, or to the rarefaction of the wax by the warmth of the weather, or
of the hand that let them fall; or, lastly, whether it proceeded from some
insensible errors in weighing the globes in the water, I am not certain.
Therefore the weight of the globe in water should be of several grains, that
the experiment may be certain, and to be depended on.

Exper. 4. I began the foregoing experiments to investigate the resistan¬
ces of fluids, before I was acquainted with the theory laid down in the
Propositions immediately preceding. Afterward, in order to examine the
theory after it was discovered, I procured a wooden vessel, whose breadth
on the inside was 8f inches, and its depth 15 feet and }. Then I made
four globes of wax, with lead included, each of which weighed 139 } grains
in air, and 7 } grains in water. These I let fall, measuring the times of their
falling in the water with a pendulum oscillating to half seconds. The
globes were cold, and had remained so some time, both when they were

OF NATURAL PHILOSOPHY.

Sec. VII.]

3l'j

weighed and when they were let fall; because warmth rarefies the wax. and
by rarefying it diminishes the weight of the globe in the water ; and wax,
when rarefied, is not instantly reduced by cold to its former density. Be¬
fore they were let fall, they were totally immersed under water, lest, by the
weight of any part of them that might chance to be above the water, their
descent should be accelerated in its beginning. Then, when after their
immersion they were perfectly at rest, they were let go with the greatest
care, that they might not receive any impulse from the hand that let them
down. And they fell successively in the times of 47^, 4S^, 50, and 51 os¬
cillations, describing a height of 15 feet and 2 inches. But the weather
was now a little colder than when the globes were weighed, and therefore 1
repeated the experiment another day; and then the globes fell in the times
of 49, 49^, 50, and 53; and at a third trial in the times of 49^, 50, 51,
and 53 oscillations. And by making the experiment several times over, I
found that the globes fell mostly in the times of 49| and 50 oscillations.
When they fell slower, I suspect them to have been retarded by striking
against the sides of the vessel.

Now, computing from the theory, the weight of the globe in vacuo is
139| grains; the excess of this weight above the weight of the globe in
water 132|i grains; the diameter of the globe 0,99868 of an inch; f parts
of the diameter 2,66315 inches; the space 2F 2,8066 inches: the space
which a globe weighing 7\ grains falling without resistance describes in a
second of time 9,SS164 inches; and the time G0",376S43. Therefore the
globe with the greatest velocity with which it is capable of descending
through the water by the force of a weight of 7} grains, will in the time
0",376843 describe a space of 2,S066 inches, and in one second of time a
space of 7,44766 inches, and in the time 25", or in 50 oscillations, the space
186,1915 inches. Subduct the space 1,386294F, or 1,9454 inches, and
there will remain the space 184,2461 inches which the globe will describe
in that time in a very wide vessel. Because our vessel was narrow, let this
space be diminished in a ratio compounded of the subduplicate ratio of the
orifice of the vessel to the excess of this orifice above half a great circle of
the globe, and of the simple ratio of the same orifice to its excess above a
great circle of the globe; and we shall have the space of 181,86 inches,
which the globe ought by the theory to describe in this vessel in the time
of 50 oscillations, nearly. But it described the space of 182 inches, by
experiment, in 49 £ or 50 oscillations.

Exper. 5. Pour globes weighing 154f grains in air, and 21| grams in
water, being let fall several times, fell in the times of 28 J, 29, 29 J, and 30,
and sometimes of 31, 32, and 33 oscillations, describing a height of 15 feet
and 2 inches.

They ought by the theory to have fallen in the time of 29 oscillations,
nearly.

350

THE MATHEMATICAL PRINCIPLES

| Book I L

Exper. 6 . Five globes, weighing 212f grains in air, and 79£ in water,
being several times let fall, fell in the times of 15, 15J, 16, 17, and 18 os¬
cillations, describing a height of 15 feet and 2 inches.

By the theory they ought to have fallen in the time cf 15 oscillations,
nearly.

Exper. 7. Four globes, weighing 293 } grains in air, and 35f grains in
water, being let fall several times, fell in the times of 29^ 30, 30J 31, 32,
and 33 oscillations, describing a height of 15 feet and 1 inch and

By the theory they ought to have fallen in the time of 28 oscillations,
nearly.

In searching for the cause that occasioned these globes of the same weight
and magnitude to fall, some swifter and some slower, I hit upon this; that
the globes, when they were first let go and began to fall, oscillated about
their centres; that side which chanced to be the heavier descending first,
and producing an oscillating motion. Now by oscillating thus, the globe
communicates a greater motion to the water than if it descended without
any oscillations; and by this communication loses part of its own motion
with which it should descend; and therefore as this oscillation is greater
or less, it will be more or less retarded. Besides, the globe always recedes
from that side of itself which is descending in the oscillation, and by so
receding comes nearer to the sides of the vessel, so as even to strike against
them sometimes. And the heavier the globes are, the stronger this oscil¬
lation is; and the greater they are, the more is the water agitated by it.
Therefore to diminish this oscillation of the globes 1 made new ones of
lead and wax, sticking the lead in one side of the globe very near its sur¬
face ; and I. let fall the globe in such a manner, that, as near as possible,
the heavier side might be lowest at the beginning of the descent. By this
means the oscillations became much less than before, and the times in which
the globes fell were not so unequal: as in the following experiments.

Exper. 8. Four globes weighing 139 grains in air, and 6| in water,
were let fall several times, and fell mostly in the time of 51 oscillations,
never in more than 52, or in fewer than 50, describing a height of 1S2
inches.

By the theory they ought to fall in about the time of 52 oscillations

Exper. 9. Four globes weighing 273£ grains in air, and 140f in water,
being several times let fall, fell in never fewer than 12, and never more
than 13 oscillations, describing a height of 182 inches.

• These globes by the theory ought to have fallen in the time of 11J- os¬
cillations, nearly.

Exper. 10. Four globes, weighing 3S4 grains in air, and 119| in water,
oeing let fall several times, fell in the times of 17£ IS, 1S£, and 19 oscilla*
tions, descril ing a height of 181J inches. And when they fell in the time

OF NATURAL PHILOSOPHY.

351

Sec. VII.]

of 19 oscillations, I sometimes heard them hit against the 3 ides of the ves¬
sel before they reached the bottom.

By the theory they ought to have fallen in the time of 1 of oscillations,
nearly.

Exper. 11. Three equal globes, weighing 43 grains in the air, and 3||
in water, being several times let fall, fell in the times of 43|, 44, 44^, 45,
and 46 oscillations, and mostly in 44 and 45. describing a height of 182J
inches, nearly.

By the theory they ought to have fallen in the time of 46 oscillations
and f, nearly.

Exper. 12. Three equal globes, weighing 141 grains in air, and 4f in
water, being let fall several times, fell in the times of 61, 62, 63, 64, and
65 oscillations, describing a space of 182 inches.

And by the theory they ought to have fallen in 64i oscillations
nearly.

From these experiments it is manifest, that when the globes fell slowly,
as in the second, fourth, fifth, eighth, eleventh, and twelfth experiments,
the times of falling are rightly exhibited by the theory ; but when the
globes fell more swiftly, as in the sixth, ninth, and tenth experiments, the
resistance was somewhat greater than in the duplicate ratio of the velocity.
For the globes in falling oscillate a little; and this oscillation, in those
globes that are light and fall slowly, soon ceases by the weakness of the
motion ; but in greater and heavier globes, the motion being strong, it con¬
tinues longer, and is not to be checked by the ambient water till after sev¬
eral oscillations Besides, the more swiftly the globes move, the less are
they pressed by the fluid at their hinder parts; and if the velocity be.per¬
petually increased, they will at last leave an empty space behind them,
unless the compression of the fluid be increased at the same time. For the
compression of the fluid ought to be increased (by Prop. XXXII and
XXXIIl)in the duplicate ratio of the velocity, in order to preserve the re¬
sistance in the same duplicate ratio. But because this is not done, the
globes that move swiftly are not so much pressed at their hinder parts as
the others; and by the defect of this pressure it comes to pass that their
resistance is a little greater than in a duplicate ratio of their velocity.

So that the theory agrees with the phaenomena of bodies falling in water
It remains that we examine the phaenomena of bodies falling in air.

Exper. 13. From the top of St. Paul's Church in London , in Juut
1710, there were let fall together two glass globes, one full of quicksilver,
the other of air; and in their fall they described a height of 220 English
feet. A wooden table was suspended upon iron hinges on one sidi, and the
other side of the same was supported by a wooden pin. The twn globes
lying upon this table were let fall together by pulling out the pin by
means of an iron wire reaching from thence quite down to the ground; S'

352 THE MATHEMATICAL PRINCIPLES [BOOK II,

that, the pin being removed, the table, which had then no support but the
iron hinges, fell downward, and turning round upon the hinges, gave leave
to the globes to drop off from it. At the same instant, with the same pull
of the iron wire that took out the pin, a pendulum oscillating to seconds
was let go, and began to oscillate. The diameters and weights of the
globes, and their times of falling, are exhibited in the following table.

The glob

Weights.

es filled with mere
Diani' ters

ury.

Times i.
falli net.

The globes full of

Weighs j Diameters.

%ir.

Times in
falling

908 grains
983

866

747

808

784

0.8 of an inch
0,8

0,8

0,75

0.75

o!75

4"

4 —

4

4 +

4

4 +

510 grains
642

599

515

483

641

5.1 inches

5.2

5.1

5,0

5,0

5.2

/

00 00 00 00 00 00

But the times observed must be corrected; for the globes of mercury (by
Galileo's theory), in 4 seconds of time, will describe 257 English feet, and
220 feet in only 3"42'". So that the wooden table, when the pin was taken
out,did not turn upon its hinges so quickly as it ought to have done; and
the slowness of that revolution hindered the descent of the globes at the
beginning. For the globes lay about the middle of the table, and indeed
were rather nearer to the axis upon which it turned than to the pin. And
hence the times of falling were prolonged about IS'"; and therefore ought
to be corrected by subducting that excess, especially in the larger globes,
which, by reason of the largeness of their diameters, lay longer upon the
revolving table than the others. This being done, the times in which the
six larger globes fell will come forth 8" 12'", 7" 42'", 7" 42'", 7" 57'", S" 12'"
and 7" 42'".

Therefore the fifth in order among the globes that were full of air being
5 inches in diameter, and 483 grains in weight, fell in 8" 12'", describing a
space of 220 feet. The weight of a bulk of water equal to this globe is
16600grains; and the weight of an equal bulk of air is -f f £- grains, or 19 T 3 ^
grains ; and therefore the weight of the globe in vacuo is 502 T 3 ¥ grains;
and this weight is to the weight of a bulk of air equal to the globe as
502 t 3 ¥ to 19 T 3 ¥ ; and so is 2F to f of the diameter of the globe, that is, to
13i inches. Whence 2F becomes 28 feet 11 inches. A globe, falling in
vacuo with its whole weight of 502 T 3 ¥ grains, will in one second of time
describe 193£ inches as above; and with the weight of 483 grains will de¬
scribe 1S5,905 inches; and with that weight 4S3 grains in vacuo will de¬
scribe the space F, or 14 feet 5\ inches, in the time of 57'" 58"", and ac¬
quire the greatest velocity it is capable of descending with in the air.
With this velocity the globe in S" 12'" of time will describe 245 feet and
5i inches. Subduct 1,3863F, or 20 feet and i an inch, and there remain
225 feet 5 inches. This space, therefore, the falling globe ought by the

Sec. YII.1

OF NATURAL philosophy

353

theory to describe in 8" 12'". But* by the experiment it deserved a space
of 220 feet. The difference is insensible.

By like calculations applied to the other globes full of air, I composed
the following table.

The weight®
of the globe

The diame¬
ters

l't.e tjmes ol
allng from!
a height u
•2-20 feet

T> e space* which they
wool l descnhe by the
heory

The excesses.

510 grain: 5

5.1 inches

8"

12'"

226 feet 11 inch.

6 feet

11 _nch.

642

5,2

7

42

230 9

10

9

599

5,1

7

42

227 10

7

0

515

5

7

57

224 5

4

5

483

5

8

12

[225 5

5

5

641

,5,2 j

7

42

|230 7

10

7

Exper. 14. Anno 1719, in the month of July , Dr. Desaguliers made
some experiments of this kind again, by forming hogs’bladders into spheri¬
cal orbs; which was done by means of a concave wooden sphere, which the
bladders, being wetted well first, were put into. After that being blown
full of air, they were obliged to fill up the spherical cavity that contained
them; and then, when dry, were taken out. These were let fall from the
lantern on the top of the cupola of the same church, namely, from a height
of 272 feet; and at the same moment of time there was let fall a leaden
globe, whose weight was about 2 pounds troy weight. And in the mean
time some persons standing in the upper part of the church where the
globes were let fall observed the whole times of falling; and others stand¬
ing on the ground observed the differences of the times between the fall
of the leaden weight and the fall of the bladder. The times were measured
by pendulums oscillating to half seconds. And one of those that stood
upon the ground had a machine vibrating four times in one second ; and
another had another machine accurately made with a pendulum vibrating
four times in a second also. One of those also who stood at the top of the
church had a like machine; and these instruments were so contrived, that
their motions could be stopped or renewed at pleasure. Now the leaden
globe fell in about four seconds and } of time; and from the addition of
this time to the difference of time above spoken of, was collected the -vVhole
time in which the bladder was falling. The times which the five bladders
spent in falling, after the leaden globe had reached the ground, were, tfie
first time, 14}", 12}", 14 J", 17}", and 16}": and the second time, 14}", 14}",
14", 19", and 16}". Add to these 4}", the time in which the leaden globe
was falling, and the whole times in which the five bladders fell were, the
first time, 19", 17", 1S} V , 22", and 21}"; and the second time, IS}", 18}",
18}", 23}", and 21". The times observed at the top of the church were,
the first time, 19f", 17}", 18}", 22}", and 21}"; and the second time, 19",
18}", IS}", 24". and 21}". But the bladders did not always fall directly
down, but sometimes fluttered a little in the air, and waved to and fro, as

93

354

THE MATHEMATICAL PRINCIPLES

[Book J1

they were descending. And by these motions the times of their falling
were prolonged, and increased by half a second sometimes, and sometimes
by a whole second. The second and fourth bladder fell most directly the
first time, and the first and third the second time. The fifth bladder was
wrinkled, and by its wrinkles was a little retarded. I found their diame¬
ters by their circumferences measured with a very fine thread wound about
them twice. In the following table I have compared the experiments with
the theory ; making the density of air to be to the density of rain-water as
1 to 860, and computing the spaces which by the theory the globes ought
to describe in falling.

The weight -
of the bla U
ders.

The diameters

t'he times ol
falling from
a height ol
272 f. et

The spaces which by
the theory ought to
have been described
in those times

The difference be
tween the theory
and the experi¬
ments

128 grains

5,28 inches

19"

271 feet 11 in.

— Oft.

1 in.

156

5.19

17

272

04

+ 0

0 h

1374

5.3

18

272

7

+ 0

7

97d

5.26

22

277

4

+ 5

4

99 h

5'

21ft

282

0

+ 10

0

Our theory, therefore, exhibits rightly, within a very little, all the re¬
sistance that globes moving either in air or in water meet with ; whiclvap-
pears to be proportional to the densities of the fluids in globes of equal ve-
loeities and magnitudes.

In the Scholium subjoined to the sixth Section, we shewed, by experi¬
ments of pendulums, that the resistances of equal and equally swift globes
moving in air, water, and quicksilver, are as the densities of the fluids.
We here prove the same more accurately by experiments of bodies falling
in air and water. For pendulums at each oscillation excite a motion in
the fluid always contrary to the motion of the pendulum in its return ; and
the resistance arising from this motion, as also the resistance of the thread
by which the pendulum is suspended, makes the whole resistance of a pen¬
dulum greater than the resistance deduced from the experiments of falling
bodies. For by the experiments of pendulums described in that Scholium,
a globe of the same density as water in describing the length of its semi¬
diameter in air would lose the P ar ^ motion. But by the

theory delivered in this seventh Section, and confirmed by experiments of
falling bodies, the same globe in describing the same length would lose only
a part of its motion equal to T 5 V e; supposing the density of water to be
to the density of air as 860 to 1. Therefore the resistances were found
greater by the experiments of pendulums (for the reasons just mentioned)
than by the experiments of falling globes; and that in the ratio of about
4 to 3. But yet since the resistances of pendulums oscillating in air, wa¬
ter, and quicksilver, are alike increased by like causes, the proportion of
the resistances in these mediums will be rightly enough exhibited by the

Sec. YII.J

OF NATURAL PHILOSOPHY.

355

experiments of pendulums, as well as by the experiments of falling bodies.
And from all this it may be concluded, that the resistances of bodies, moving
in any fluids whatsoever, though of the most extreme fluidity, are, cccteris
paribus , as the densities of the fluids.

These things being thus established, we may now determine what part
of its motion any globe projected in any fluid whatsoever would nearly lose
in a given time. Let D be the diameter of the globe, and V its velocity
at the beginning of its motion, and T the time in which a globe with the
velocity Y can describe in vacuo a space that is, to the space |D as the
density of the globe to the density of the fluid; and the globe projected

tY

in that fluid will, in any other time t lose the part - , the part

A i £

TY

^ remaining; and will describe a space, which will be to that de¬
scribed in the same time in, vacuo with the uniform velocity Y, as the
T + t

logarithm of the number —^— multiplied by the number 2,3025S5093 is

to the number by Cor. 7, Prop. XXXV. In slow motions the resist¬
ance may be a little less, because the figure of a globe is more adapted to
motion than the figure of a cylinder described with the same diameter. In
swift motions the resistance may be a little greater, because the elasticity
and compression of the fluid do not increase in the duplicate ratio of the
velocity. But these little niceties I take no notice of.

And though air, water, quicksilver, and the like fluids, by the division
of their parts in infinitum , should be subtilized, and become mediums in¬
finitely fluid, nevertheless, the resistance they would make to projected
globes would be the same. For the resistance considered in the preceding
Propositions arises from the inactivity of the matter; and the inactivity
of matter is essential to bodies, and always proportional to the quantity
of matter. By the division of the parts of the fluid the resistance arising
from the tenacity and friction of the parts may be indeed diminished; but
the quantity of matter will not be at all diminished by this division; and
if the quantity of matter be the same, its force of inactivity will be the
same; and therefore the resistance here spoken of will be the sanue, as being
always proportional to that force. To diminish this resistance, the quan¬
tity of matter in the spaces through which the bodies move must be dimin¬
ished ; and therefore the celestial spaces, through which the globes of the
planets and comets are perpetually passing towards all parts, with the
utmost freedom, and without the least sensible diminution of their motion,
must be utterly void of any corporeal fluid, excepting, perhaps, some ex¬
tremely rare vapours and the rays of light.

356

THE MATHEMATICAL PRINCIPLES

[Book 1L

Projectiles excite a motion in fluids as they pass through them, and this
motion arises from the excess of the pressure of the fluid at the fore parts
of the projectile above the pressure of the same at the hinder parts; and
cannot be less in mediums infinitely fluid than it is in air, water, and quick¬
silver, in proportion to the density of matter in each. Now this excess of
pressure does, in proportion to its quantity, not only excite a motion in the
fluid, but also acts upon the projectile so as to retard its motion ; and there¬
fore the resistance in every fluid is as the motion excited by the projectile
in, the fluid; and cannot be less in the most subtile aether in proportion to
the density of that aether, than it is in air, water, and Quicksilver, in pro¬
portion to the densities of those fluids.

SECTION VIII.

Of motion propagated through fluids .

PROPOSITION XLI. THEOREM XXXII.

A pressure is not propagated through a fluid in rectilinear directions
unless where the particles of the fluid lie in a right line.

If the particles a, b , c, d, e, lie in a right line, the pres¬
sure may be indeed directly propagated from a to e; but
then the particle e will urge the obliquely posited parti¬
cles / and g obliquely, and those particles f and g will
not sustain this pressure, unless they be supported by the
particles h and k lying beyond them; but the particles
that support them are also pressed by them; and those particles cannot
sustain that pressure, without being supported by, and pressing upon, those
particles that lie still farther, as l and m, and so on in infinitum. There¬
fore the pressure, as soon as it is propagated to particles that lie out of
right lines, begins to deflect towards one hand and the other, and will be
propagated obliquely in infinitum ; and after it has begun to be propagat¬
ed obliquely, if' it reaches more distant particles lying out of the right
line, it will deflect again on each hand and this it will do as often as it
lights on particles that do not lie exactly in a right line. Q.E.D.

Cor. If any part of a pressure, propagated through a fluid from a given
point, be intercepted by any obstacle, the remaining part, which is not in¬
tercepted, will deflect into the spaces behind the obstacle. This may be
demonstrated also after the following manner. Let a pressure be propagat¬
ed from the point A towards any part, and, if it be possible, in rectilinear

5Ec» Vlll.J

OF NATURAL PHILOSOPHY.

35 7

directions ; and the obstacle
NBCK being perforated in BC,
let all the pressure be intercepted
but the coniform part APQ, pass¬
ing through the circular hole BC.

Let the cone APQ be divided
into frustums by the transverse
planes, de, fg, hi. Then while
the cone ABC, propagating the
pressure, urges the conic frustum
degf beyond it on the superficies
de, and this frustum urges the next frustumon the superficies/g-, and
that frustum urges a third frustum, and so in infinitum; it is manifest
(by the third Law r ) that the first frustum defg is, by the re-action of the
second frustum fghi , as much urged and pressed on the superficies fg, as
it urges and presses that second frustum. Therefore the frustum degf is
compressed on both sides, that is, between the cone Ade and the frustum
fhig; and therefore (by Case 6, Prop. XIX) cannot preserve its figure,
unless it be compressed with the same force on all sides. Therefore wuth
the same force with which it is pressed on the superficies de,fg , it will
endeavour to break forth at the sides df eg ; and there (being not in the
least tenacious or hard, but perfectly fluid) it will run out, expanding it¬
self, unless there be an ambient fluid opposing that endeavour. Therefore,
by the effort it makes to run out, it will press the ambient fluid, at its sides
df eg, with the same force that it does the frustum fghi; and therefore,
the pressure will be propagated as much from the sides df eg, into the
spaces NO, KL this way and that way, as it is propagated from the su¬
perficies fg towards PQ. Q.E.D.

PROPOSITION NLII. THEOREM XXXIII.

All motion propagated through a fluid diverges from a rectilinear pro*
gress into the unmoved spaces.

Case 1. Let a motion be
propagated from the point A
through the hole BC, and, if it
be possible, let it proceed in the
conic space BCQ,P according to
right lines diverging from the
point A. And let us first sup¬
pose this motion to be that of
waves in the surface of standing
water; and let de,fg, hi, kl, &c.,
be the tops of the several waves,
divided from each other by as
any intermediate valleys or hollows. Then, because the water in the

35S

THE MATHEMATICAL PRINCIPLES

[Book I*

ridges of the waves is higher than in the unmoved parts of the fluid'KL ;
NO, it will run down from off the tops of those ridges, e, g, i, l , (fee., d,f,
h, k, (fee., this way and that way towards KL and NO; and because the
water is more depressed in the hollows of the waves than in the unmoved
parts of the fluid KL, NO, it will run down into those hollows out of those
unmoved parts. By the first deflux the ridges of the waves will dilate
themselves this way and that way, and be propagated towards KL and NO.
And because the motion of the waves from A towards PQ is carried on by
a continual deflux from the ridges of the waves into the hollows next to
them, and therefore cannot be swifter than in proportion to the celerity of
the descent; and the descent of the water on each side towards KL and NO
must be performed with the same velocity; it follows that the dilatation
of the waves on each side towards KL and NO will be propagated with the
same velocity <is the waves themselves go forward with directly from A to
PQ,. And therefore the whole space this way and that way towards KL
and NO will be filled by the dilated waves rfgr, shis , tklt , vmnv , (fee.
Q.E.1). That these things are so, anyone may find by making the exper¬
iment in still water.

Case 2. Let us suppose that de , fg, hi, kl , ran, represent pulses suc¬
cessively propagated from the point A through an elastic medium. Con¬
ceive the pulses to be propagated by successive condensations and rarefactions
of the medium, so that the densest part of every pulse may occupy a
spherical superficies described about the centre A, and that equal intervals
intervene between the successive pulses. Let the lines de, fg, hi, kl, (fee.,
represent the densest parts of the pulses, propagated through the hole BC:
and because the medium is denser there than in the spaces on either side
towards KL and NO. it will dilate itself as well towards those spaces KL,
NO, on each hand, as towards the rare intervals between the pulses; and
thence the medium, becoming always more rare next the intervals, and
more dense next the pulses, will partake of their motion. And because the
progressive motion of the pulses arises from the perpetual relaxation of the
denser parts towards the antecedent rare intervals; and since the pulses will
relax themselves on each hand towards the quiescent parts of the medium
KL, NO, with very near the same celerity; therefore the pulses will dilate
themselves on all sides into the unmoved parts KL, NO, with almost the
same celerity with w r hich they are propagated directly from the centre A;
and therefore will fill up the whole space KLON. Q.E.D. And we find
the same by experience also in sounds which are heard through a mountain
interposed; and,*if they come into a chamber through the window, dilate
themselves into all the parts of the room, and are heard in every earner;
and not as reflected from the opposite walls, but directly propagated from
the window, as far as our sense can judge.

Case 3 Let us suppose, lastly, that a motion of any kind is propagated

OF NATURAL PHILOSOPHY.

35?

«c. VIII.J

from A through the hole BC. Then since the cause of this propagation is
that the parts of the medium that are near the centre A disturb and agitate
those which lie farther from it; and since the parts which are urged are
fluid, and therefore recede every way towards those spaces where they are
less pressed, they will by consequence recede towards all the parts of tht
quiescent medium; as well to the parts on each hand, as KL and NO,
as to those right before, as PQ,; and by this means all the motion, as soon
as it has passed through the hole BC, will begin to dilate itself, and from
thence, as from its principle and centre, will be propagated directly every
way. Q.E.D.

PROPOSITION XLIII. THEOREM XXXIV.

Every tremulous body in an elastic medium propagates the motion of
the. pulses on every side right forward ; but in a non-elastic :medium
excites a circular motion.

Case. 1. The parts of the tremulous body, alternately going and return¬
ing, do in going urge and drive before them those parts of the medium that
lie nearest, and by that impulse compress and condense x them ; and in re¬
turning suffer those compressed parts to recede again, and expand them¬
selves. Therefore the parts of the medium that lie nearest to the tremulous
body move to and fro by turns, in like manner as the parts of the tremulous
body itself do; and for the same cause that the parts of this body agitate
these parts of the medium, these parts, being agitated by like tremors, will
in their turn agitate others next to themselves; and these others, agitated
in like manner, will agitate those that lie beyond them, and so on in infin¬
itum. And in the same manner as the first parts of the medium were
condensed in going, and relaxed in returning, so will the other parts be
condensed every time they go, and expand themselves every time they re¬
turn. And therefore they will not be all going and all returning at the
same instant (for in that case they would always preserve determined dis¬
tances from each other, and there could be no alternate condensation and
rarefaction); but since, in the places where they are condensed, they ap¬
proach to, and, in the places where they are rarefied, recede from each other,
therefore some of them will be going while others are returning; and so on
in infinitum. The parts so going, and in their going condensed, are pulses,
by reason of the progressive motion with which they strike obstacles in
their way; and therefore the successive pulses produced by a tremulous
body will be propagated in rectilinear directions; and that at nearly equal
distances from each other, because of the equal intervals of time in which
the body, by its several tremors produces the several pulses. And though
the parts of the tremulous body go and return .n some certain and deter¬
minate direction, yet the pulses propagated from thence through the medium
will dilate themselves towards the sides, by the foregoing Proposition; and

360 the mathematical principles [Book 11

will be propagated on all sides from that tremulous body, as from a com¬
mon centre, in superficies nearly spherical and concentrical. An example
of this we have in waves excited by shaking a finger in water, which
proceed not only forward and backward agreeably to the motion of the
finger, but spread themselves in the manner of concentrical circles all round
the finger, and are propagated on every side. For the gravity of the water
supplies the place of elastic force.

Case 2. If the medium be not elastic, then, because its parts cannot be
condensed by the pressure arising from the vibrating parts of the tremulous
body, the motion will be propagated in an instant towards the parts where
the medium yields most easily, that is, to the parts which the tremulous
body would otherwise leave vacuous behind it. The case is the same with
that of a body projected in any medium whatever. A medium yielding
to projectiles does not recede in infinitum , but with a circular motion comes
round to the spaces which the body leaves behind it. Therefore as often
as a tremulous body tends to any part, the medium yielding to it comes
round in a circle to the parts which the body leaves; and as often as the
body returns to the first place, the medium will be driven from the place it
came round to, and return to its original place. And though the tremulous
bod} be not firm and hard, but every way flexible, yet if it continue of a
given magnitude, since it cannot impel the medium by its tremors any
where without yielding to it somewhere else, the medium receding from the
parts of the body where it is pressed will 'always come round in a circle to
the parts that yield to it. Q.E.D.

Cor. It is a mistake, therefore, to think, as some have done, that the
agitation of the parts of flame conduces to the propagation of a pressure in
rectilinear directions through an ambient medium. A pressure of that
kind must be derived not from the agitation only of the parts of flame, but
from the dilatation of the whole.

PROPOSITION XL1V. THEOREM XXXV.

If water ascend and descend alternately in the erected legs KL , MN, of
a canal or pipe ; and a pendidum be constructed whose length between
the point of suspension and the centre of oscillation is equal to half
the length of the water in the canal ; I say , that the water trill ascend
and descend in the same times in ichich the pendulum oscillates .

I measure the length of the water along the axes of the canal and its legs,
and make it equal to the sum of those axes; and take no notice of the
resistance of the water arising from its attrition by the sides of the canal.
Let, therefore, AB, CD, represent the mean height of the water in both
legs; and when the water in the leg KL ascends to the height EF, the
water will descend in the leg MN to the height GH. Let P be a pendulour

Sec. Vlil.J of natural philosophy. 361

body, YP the thread, V the point of suspension, RPQS the cycloid which

the pendulum describes, P its lowest point, PQ an arc equal to the neiglit
AE. The force with which the motion of the water is accelerated and re¬
tarded alternately is the excess of the weight of the water in one leg above
the weight in the other; and, therefore, when the water in the leg KL
ascends to EF, and in the other leg descends to GH, that force is double
the weight of the water EABF, and therefore is to the weight of the whole
water as AE or PQ, to VP or PR. The force also with which the body P
is accelerated or retarded in any place, as Q, of a cycloid, is (by Cor. Prop.
LI) to its whole weight as its distance PQ from the lowest place P to the
length PR of the cycloid. Therefore the motive forces of the water and
pendulum, describing the equal spaces AE, PQ, are as the weights to be
moved; and therefore if the water and pendulum are quiescent at first,
those forces will move them in equal times, and will cause them to go and
return together with a reciprocal motion. Q.E.D.

Cor. 1. Therefore the reciprocations of the water in ascending and de¬
scending are all performed in equal times, whether the motion be more or
less intense or remiss.

Cor. 2. If the length of the whole water in the canal be of 6J feet ol
French measure, the water will descend in one second of time, and will as¬
cend in another second, and so on by turns in infinitum; for a pendulum
of 3^ such feet in length will oscillate in one second of time.

Cor. 3. But if the length of the water be increased or diminished, the
time of the reciprocation will be increased or diminished in the subdupli¬
cate ratio of the length.

PROPOSITION XLY. THEOREM XXXVI.

The velocity of leaves is in the subduplicate ratio of the breadths.

This follows from the construction of the following Proposition.

PROPOSITION XLVI. PROBLEM X.

To find the velocity of waves.

Let a pendulum be constructed, whose length between the point of sus¬
pension and the centre of oscillation is equal to the breadth of the waves

362

THE MATHEMATICAL PRINCIPLES

[Book 1L

and in the time that the pendulum will perform one single oscillation the
waves will advance forward nearly a space equal to their breadth.

That which I call the breadth of the waves is the transverse measure

lying between the deepest
part of the hollows, or the
tops of the ridges. Let
ABCDEF represent the surface of stagnant water ascending and descend¬
ing in successive waves; and let A, C, E, (fee., be the tops of the waves;
find let B, D, F, (fee., be the intermediate hollows. Because the motion of'
the waves is carried on by the successive ascent and descent of the water,
so that the parts thereof, as A, C, E, (fee., which are highest at one time
become lowest immediately after; and because the motive force, by which
the highest parts descend and the lowest ascend, is the weight of the eleva¬
ted water, that alternate ascent and descent will be analogous to the recip¬
rocal motion of the water in the canal, and observe the same laws as to the
times of its ascent and descent; and therefore (by Prop. XL1V) if the
distances between the highest places of the waves A, C, E, and the lowest
B, D, F, be equal to twice the length of any pendulum, the highest parts
A, C, E, will become the lowest in the time of one oscillation, and in the
time of another oscillation will ascend again. Therefore between the pas¬
sage of each wave, the time of two oscillations will intervene; that is, the
wave will describe its breadth in the time that pendulum will oscillate
twice; but a pendulum of four times that length, and which therefore is
equal to the breadth of the waves, will just oscillate once in that time.
Q.E.I.

Cor. 1. Therefore waves, whose breadth is equal to 3^ French feet,
will advance through a space equal to their breadth in one second of time;
and therefore in one minute will go over a space of 183J feet; and in an
hour a space of 11000 feet, nearly.

Cor. 2. And the velocity of greater or less waves will be augmented or
diminished in the subduplicate ratio of their breadth.

These things are true upon the supposition that the parts of water as¬
cend or descend in a right line; but, in truth, that ascent and descent is
rather performed in a circle; and therefore I propose the time defined by
this Proposition as only near the truth.

PROPOSITION XLVII. THEOREM XXXVII.

If pulses are propagated through a fluid , the se eral particles of the
fluid , going and returning with the shortest reciprocal motion , are al¬
ways accelerated or retarded according to the law of the oscillating
pendulum.

Let AB, BC, CD, (fee., represent equal distances of successive pulses,
ABC the line of direction of the motion of the successive pulses propagated

OF NATURAL PHILOSOPHY.

Sec. VIIL]

303

&

SI

from A to B; E, F, G three physical points of the quiescent medium sit¬
uate in the right line AC at equal distances from each other; Ee, F f, Gg t
equal spaces of extreme shortness, through which those
points go and return with a reciprocal motion in each vi¬
bration ; e, 0, y, any intermediate places of the same points;

EF, FG physical lineolae, or linear parts of the medium
lying between those points, and successively transferred into
the places <-.0, 0y, and ef, fg. Let there be drawn the
right line PS equal to the right line Ee. Bisect the same
in O, and from the centre O, with the interval OP, describe
the circle SIPi. Let the whole time of one vibration ; with
its proportional parts, be expounded by the whole circum-
lerence of this circle and its parts, in such sort, that, when
any time PH or PHSA is completed, if there be let fall to
PS the perpendicular HL or hi , and there
be taken Ee equal to PL or P/, the physi¬
cal point E may be found in e. A point,
as E, moving acccording to this law with
a reciprocal motion in its going from E
through e to e, and returning again through
e to E, will perform its several vibrations with the same de¬
grees of acceleration and retardation with those of an oscil¬
lating pendulum. We are now to prove that the several
physical points of the medium will be agitated with such a
kind of motion. Let us suppose, then, that a medium hath
such a motion excited in it from any cause whatsoever, and
consider what will follow from thence.

In the circumference PHSA let there be taken the equal
arcs, HI, IK, or hi, ik, having the same ratio to the whole
circumference as the equal right lines EF, FG have to BC,
the whole interval of the pulses. Let fall the perpendicu¬
lars IM, KN, or im, kn ; then because the points E, F, G are
successively agitated with like motions, and perform their entire vibrations
composed of their going and return, while the pulse is transferred from B
to C; if PH or PHSA be the time elapsed since the beginning of the mo¬
tion of the point E, then will PI or PHSi be the time elapsed since the
beginning of the motion of the point F, and PK or PHS& the time elapsed
since the beginning of the motion of the point G; and therefore Ee, F0,
Gy, will be respectively equal to PL, PM, PN, while the points are going,
and to VI, P m, P n, when the points are returning. Therefore ey or EG
Gy — Ee will, when the points are going, be equal to EG — LN

ill

I i

364

THE MATHEMATICAL PRINCIPLES

[Book II.

and in their return equal to EG + In. But ey is the breadth or ex¬
pansion of the part EG of the medium in the place ey ; and therefore the
expansion of that part in its going is to its mean expansion as EG —
LN to EG; and in its return, as EG 4 - la or EG -f LN to EG.
Therefore since LN is to KH as IM to the radius OP, and KH to EG
as the circumference PHSAP to BC; that is, if we put V for the
radius of a circle whose circumference is equal to BC the interval of the
pulses, as OP to V; and, ex cequo , LN to EG as IM to V; the expansion
of the part EG, or of the physical point P in the place ey, to the mean ex¬
pansion of the same part in its first place EG, will be as V — IM to V
in going, and as V + ini to V in its return. Hence the elastic force of the
point F in the place ey to its mean elastic force in the place EG is as

77 -r— to T in its going, and as —-—.— to ^ in its return. And by

the same reasoning the elastic forces of the physical points E and G in going
are as ^ —tvt- and m to ; and the difference of the forces to the

y —hl

V — KN V

mean elastic force of the medium as „

yy-y x hl-vx Kjn + hl x kn

_KN 1

to =, or as HL — KN to V ; if we suppose

to y : that is, as

yy

(by reason of the very short extent of the vibrations) HL and KN to be
indefinitely less than the quantity y. Therefore since the quantity V is
given, the difference of the forces is as HL — KN ; that is (because HL
— KN is proportional to HK, and OM to 01 or OP; and because HK
and OP are given) as OM; that is, if Ff be bisected in £ 2 , as S 20 . And
fur the same reason the difference of the elastic forces of the physical points
e and y, in the return of the physical lineola ey, is as Qd>. But that dif¬
ference (that is, the excess of the elastic force of the point £ above the
elastic force of the point y) is the very force by which the intervening phy¬
sical lineola ey of the medium is accelerated in going, and retarded in re¬
turning ; and therefore the accelerative force of the physical lineola ey is
as its distance from Q, the middle place of the vibration. Therefore (by
Prop. XXXVIII, Book 1) the time is rightly expounded by the arc PI;
and the linear part of the medium ey is moved according to the law above-
mentioned, that is, according to the law of a pendulum oscillating; and
the case is the same of all the linear parts of which the whole medium is
compounded. QJE.D.

Cor. Hence it appears that the number of the pulses propagated is the
same with the number of the vibrations of the tremulous body, and is not
multiplied in their progress. For the physical lineola ty as soon as it
returns to its first place is at rest; neither will it move again, unless it

Sec. Y11I.J

OF NATURAL PHILOSOPHY.

365

receives a new motion either from the impulse of the tremulous body, or
of the pulses propagated from that body. As soon, therefore, as the pulses
cease to be propagated from the tremulous body, it will return to a state
of rest, and move no more.

PROPOSITION XLVIII. THEOREM XXXVIII.

The velocities of pulses propagated in an elastic fluid are in a ratin
compounded of the subduplicate ratio of the elastic force directly , and
the subduplicate ratio of the density inversely; supposing the elastic
jorce of the fluid to be proportional to its condensation
Case 1. If the mediums be homogeneous, and the distances of the pulses
in those mediums be equal amongst themselves, but the motion in one me¬
dium is more intense than in the other, the contractions and dilatations of
the correspondent parts will be as those motions; not that this proportion
is perfectly accurate. However, if the contractions and dilatations are not
exceedingly intense, the error will not be sensible ; and therefore this pro¬
portion may be considered as physically exact. Now the motive elastic
forces are as the contractions and dilatations ; and the velocities generated
in the same time in equal parts are as the forces. Therefore equal and
corresponding parts of corresponding pulses will go and return together,
through spaces proportional to their contractions and dilatations, with ve¬
locities that are as those spaces; and therefore the pulses, which in the
time of one going and returning advance forward a space equal to their
breadth, and are always succeeding into the places of the pulses that im¬
mediately go before them, will, by reason of the equality of the distances,
go forward in both mediums with equal velocity.

Case 2. If the distances of the pulses or their lengths are greater in one
medium than in another, let us suppose that the correspondent parts de¬
scribe spaces, in going and returning, each time proportional to the breadths
of the pulses ; then will their contractions and dilatations be equal: and
therefore if the mediums are homogeneous, the motive elastic forces, which
agitate them with a reciprocal motion, will be equal also. Now the matter
to be moved by these forces is as the breadth of the pulses; and the space
through which they move every time they go and return is in the same
ratio. And, moreover, the time of one going and returning is in a ratic
compounded of the subduplicate ratio of the matter, and the o-ubdupncate
ratio of the space ; and therefore is as the space. But the pulses advance
a space equal to their breadths in the times of going once and returning
once; that is, they go over spaces proportional to the times, and therefore
are equally swift.

Case 3. And therefore in mediums of equal density and elastic force,
all the pulses are equally swift. Now if the density or the elastic force of
the medium were augmented, then, because the motive force is increased

366

THE MATHEMATICAL PRINCIPLES

[Book 11

in the ratio of the elastic force, and the matter to be moved is increased in
the ratio of the density, the time which is necessary for producing the
same motion as before will be increased in the subduplicate ratio of the
density, and will be diminished in the subduplicate ratio of the elastic
force. And therefore the velocity of the pulses will be in a ratio com¬
pounded of the subduplicate ratio of the density of the medium inversely,
and the subduplicate ratio of the elastic force directly. Q,.E.D.

This Proposition will be made more clear from the construction of the
following Problem.

PROPOSITION XLIX. PROBLEM XI.

The density and elastic force of a medium being given , to find the ve¬
locity of the pulses.

Suppose the medium to be pressed by an incumbent weight after the manner
of our air; and let A be the height of a homogeneous medium, whose
weight is equal to the incumbent weight, and whose density is the same
with the density of the compressed medium in which the pulses are propa¬
gated. Suppose a pendulum to be constructed whose length between the
point of suspension and the centre of oscillation is A: and in -the time in
which that pendulum will perform one entire oscillation composed of
its going and returning, the pulse will be propagated right onwards
through a space equal to the circumference of a circle described with the
radius A.

For, letting those things stand which were constructed in Prop. XLVI1,
if any physical line, as EF, describing the space PS in each vibration, be
acted on in the extremities P and S of every going and return that it
makes by an elastic force that is equal to its weight, it will perform its
several vibrations in the time in which the same might oscillate in a cy¬
cloid whose whole perimeter is equal to the length PS ; and that because
equal forces will impel equal corpuscles through equal spaces in the same
or equal times. Therefore since the times of the oscillations are in the
subduplicate ratio of the lengths of the pendulums, and the length of the
pendulum is equal to half the arc of the whole cycloid, the time of one vi¬
bration would be to the time of the oscillation of a pendulum whose length
is A in the subduplicate ratio of the length IPS or PO to the length A.
But the elastic force with which the physical lineola EG is urged, when it
Is found in its extreme places P, S, was (in the demonstration of Prop.
XLYII) to its whole elastic force as HL — KN to Y, that is (since the
point K now falls upon P), as HK to Y: and all that force, or which is
the same thing, the incumbent weight by which the lineola EG is com¬
pressed, is to the weight of the lineola as the altitude A of the incumbent
weight to EG the length of the lineola; and therefore, ex aquo , the force

Sec. VIILJ

OF NATURAL PHILOSOPHY.

367

J e L

n

y

X

T

li

1

lii

iii

Ml

1,

II

i

with which the lineola EG is urged in the places P and S
is to the weight of that lineola as HK X A to V X EG ; or
as PO X A to YY; because HK was to EG as PO to V.

Therefore since the times in which equal bodies are impelled
through equal spaces are reciprocally in the subduplicate
ratio of the forces, the time of one vibration, produced by
the action of that elastic force, will be to the time of a vi¬
bration, produced by. the impulse of the weight in a subdu¬
plicate ratio of yy to PO X A, and therefore to the time
of the oscillation of a pendulum whose length is A in the
subduplicate ratio of yy to PO X A, and the subdupli¬
cate ratio of PO to A conjunctly; that is, in the entire ra¬
tio of y to A. But in the time of one
vibration composed of the going and re¬
turning of the pendulum, the pulse will
be propagated right onward through a
space equal to its breadth BC. There¬
fore the time in which a pulse runs over
the space BC is to the time of one oscillation composed of
the going and returning of the pendulum as V to A, that is,
as BC to the circumference of a circle whose radius is A.

But the time in which the pulse will run over the space BC
is to the time in which it will run over a length equal to
that circumference in the same ratio; and therefore in the
time of such an oscillation the pulse will run over a length
equal to that circumference. Q,.E.D.

Cor. 1. The velocity of the pulses is equal to that which
heavy bodies acquire by falling with an equally accele¬
rated motion, and in their fall describing half the alti¬
tude A. For the pulse will, in the time of this fall, sup¬
posing it to move with the velocity acquired by that fall, run over a
space that will be equal to the whole altitude A; and therefore in the
time of one oscillation composed of one going and return, will go over a
space equal to the circumference of a circle described with the radius A;
for the time of the fall is to the time of oscillation as the radius of a circle
to its circumference.

Cor. 2. Therefore since that altitude A is as the elastic force of the
fluid directly, and the density of the same inversely, the velocity of the
pulses will be in a ratio compounded of the su}>duplicate ratio of the den¬
sity inversely, and the subduplicate ratio of the clastic force directly.

363

THE MATHEMATICAL PRINCIPLES

[Book IL

PROPOSITION L. PROBLEM XII.

To find the distances of the pulses.

Let the number of the vibrations of the body, by whose tremor the pulses
are produced, be found to any given time. By that number divide the
space which a pulse can go over in the same time, and the part found will
be the breadth of one pulse. Q.E.I.

SCHOLIUM.

The last Propositions respect the motions of light and sounds; for since
light is propagated in right lines, it is certain that it cannot consist in ac-
— tion alone (by Prop. XLI and XLIl). As to sounds, since they arise from
tremulous bodies, they can be nothing else but pulses of the air propagated
through it (by Prop. XLIII); and this is confirmed by the tremors which
sounds, if they be loud and deep, excite in the bodies near them, as we ex¬
perience in the sound of drums; for quick and short tremors are less easily
excited. But it is well known that any sounds, falling upon strings in
unison with the sonorous bodies, excite tremors in those strings. This is
also confirmed from the velocity of sounds; for since the specific gravities
of rain-water and quicksilver are to one another as about 1 to 131, and
when the mercury in the barometer is at the height of 30 inches of our
measure, the specific gravities of the air and of rain-water are to one
another as about 1 to 870, therefore the specific gravity of air and quick¬
silver are to each other as 1 to 11890. Therefore when the height of
the quicksilver is at 30 inches, a height of uniform air, whose weight would
be sufficient to compress our air to the density we find it to be of, must be
equal to 356700 inches, or 29725 feet of our measure; and this is that
very height of the medium, which I have called A in the construction of
the foregoing Proposition. A circle whose radius is 29725 feet is 1S676S
feet in circumference. And since a pendulum 39} inches in length com¬
pletes one oscillation, composed of its going and return, in two seconds of
time, as is commonly known, it follows that a pendulum 29725 feet, or
356700 inches in length will perform a like oscillation in 190£ seconds.
Therefore in that time a sound will go right onwards 18676S feet, and
therefore in one second 979 feet.

But in this computation we have made no allowance for the crassitude
of the solid particles of the air, by which the sound is propagated instan¬
taneously. Because the weight of air is to the weight of water as 1 tc
870, and because salts are almost twice as dense as water; if the particles
of air are supposed to be of near the same density as those of water or salt,
and the rarity of the air arises from the intervals of the particles; the
diameter of one particle of air will be to the interval between the centres

Sec. VIII.]

OF NATURAL PHILOSOPHY.

369

of the particles as 1 to about 9 or 10, and to the interval between the par¬
ticles themselves as 1 to 8 or 9. Therefore to 979 feet, which, according to
the above calculation, a sound will advance forward in one second of time,
we may add or about 109 feet, io compensate for thecra-ssitude of the
particles of the air: and then a sound will go forward about 10S8 feet in
one second of time.

Moreover, the vapours floating in the air being of another spring, and a
different tone, will hardly, if at all, partake of the motion of the true air
in which the sounds are propagated. Now if these vapours remain unmov¬
ed, that motion will be propagated the swifter through the true air alone,
and that in the subduplicate ratio of the defect of the matter. So if the
atmosphere consist of ten parts of true air and one part of vapours, the
motion of sounds will be swifter in the subduplicate ratio of 11 to 10, or
very nearly in the entire ratio of 21 to 20, than if it were propagated
through eleven parts of true air : and therefore the motion of sounds above
discovered must be increased in that ratio. By this means the sound will
pass through 1142 feet in one second of time.

These things will be found true in spring and autumn, when the air is
rarefied by the gentle warmth of those seasons, and by that means its elas¬
tic force becomes somewhat more intense. But in winter, when the air is
condensed by the cold, and its elastic force is somewhat remitted, the mo¬
tion of sounds will be slower in a subduplicate ratio of the density ; and,
on the other hand, swifter in the summer.

Now by experiments it actually appears that sounds do really advance
in one second of time about 1142 feet of English measure, or 1070 feet of
French measure.

The velocity of sounds being known, the intervals of the pulses are known
also. For M. Sauveur , by some experiments that he made, found that an
open pipe about five Paris feet in length gives a sound of the same tone
with a viol-string that vibrates a hundred times in one second. Therefore
there are near 100 pulses in a space of 1070 Paris feet, which a sound runs
over in a second of time; and therefore one pulse fills up a space of about 1 O t 7 - 0
Paris feet, that is, about twice the length of the pipe. From whence it is
probable that the breadths of the pulses, in all sounds made in open pipes,
are equal to twice the length of the pipes.

Moreover, from the Corollary of Prop. XLVIl appears the reason why
the sounds immediately cease with the motion of the sonorous body, and
why they are heard no longer when we are at a great distance from the
sonorous bodies than when we are very near them. And besides, from the
foregoing principles, it plainly appears how it comes to pass that sounds are
so mightily increased in speaking-trumpets; for all reciprocal motion uses
to be increased by the generating cause at each return. And in tubes hin¬
dering the dilatation of the sounds, the motion decays more slowly, and

24

370

THE MATHEMATICAL PRINCIPLES

[Book II.

recurs more forcibly; and therefore is the more increased by the new mo¬
tion impressed at each return. And these are the principal phaen )mena of
sounds.

SECTION IX.

Of the circular motion of fluids .

HYPOTHESIS.

The resistance arising from the leant of lubricity in the parts of a fluid,
is, caeteris paribus, proportional to the velocity with which the parts of
the fluid are separated from each other .

PROPOSITION LI. THEOREM XXXIX.

If a solid cylinder infinitely long, in an uniform and infinite fluid, revolve
with an uniform motion about an axis given in position, and the fluid
be forced round by only this impulse of the cylinder, and every part
of the fluid persevere uniformly in its motion ; I say, that the periodic
times of the parts of the fluid are as their distances from the axis of
the cylinder.

Let AFL be a cylinder turning uni¬
formly about the axis S, and let the
concentric circles BGM, CHN, DIO,
EKP, &c., divide the fluid into innu¬
merable concentric cylindric solid orbs
of the same thickness. Then, because
the fluid is homogeneous, the impres¬
sions which the contiguous orbs make
upon each other mutually will be (by
the Hypothesis) as their translations
from eacl\ other, and as the contiguous
superficies upon which the impressions
are made. If the,impression made upon any orb be greater or less on its
concave than on its convex side, the stronger impression will prevail, and
will either accelerate or retard the motion of the orb, according as it agrees
with, or is contrary to, the motion of the same. Therefore, that every orb
may persevere uniformly in its motion, the impressions made on both sides
must be equal and their directions contrary. Therefore since the impres¬
sions are as the contiguous superficies, and as their translations from one
another, the translations will be inversely as the superficies, that is, inversely
as the distances of the superficies from the axis. But the differences of

Sec. IX.1

OF NATURAL PHILOSOPHY.

37 1

the angular motions about the axis are as those translations applied to the
distances, or as the translations d.rectly and the distances inversely; that
is, joining these ratios together, as the squares of the distances inversely.
Therefore if there be erected the lines An, Bb, Cc, Dd, Ee, &c., perpendic¬
ular to the several parts of he infinite right line SABCDEQ,, and recip¬
rocally proportional to the squares of SA, SB, SC, SD, SE, &c., and
through the extremities of those perpendiculars there be supposed to pass
an hyperbolic curve, the sums of the differences, that is, the whole angular
motions, will be as the correspondent sums of the lines Aa , B b, Cc, Dd, Ee,
that is (if to constitute a medium uniformly fluid the number of the orbs
be increased and their breadth diminished in infinitum), as the hyperbolic
areas AaQ,, B&Q,, CcQ,, Dt/Q,, EeQ, &c., analogous to the sums; and the
times, reciprocally proportional to the angular motions, will be also recip¬
rocally proportional to those areas. Therefore the periodic time of any
particle as D, is reciprocally as the area Dc/Q,, that is (as appears
from the known methods of quadratures of curves), directly as the dis¬
tance SD. Q.E.D.

Cor. 1. Hence the angular motions of the particles of the fluid are re
ciprocally as their distances from the axis of the cylinder, and the absolute
velocities are equal.

Cor. 2. If a fluid be contained in a cylindric vessel of an infinite length,
and contain another cylinder within, and both the cylinders revolve about
one common axis, and the times of their revolutions be as their semi¬
diameters, and every part of the fluid perseveres in its motion, the peri¬
odic times of the several parts will be as the distances from the axis of the
cylinders.

Cor. 3. If there be added or taken away any common quantity of angu¬
lar motion from the cylinder and fluid moving in this manner; yet because
this new motion will not alter the mutual attrition of the parts of the fluid,
the motion of the parts among themselves will not be changed; for the
translations of the parts from one another depend upon the attrition.
Any part will persevere in that motion, which, by the attrition made
on both sides with contrary directions, is no more accelerated than it is re¬
tarded.

Cor. 4. Therefore if there be taken away from this whole system of the
cylinders and the fluid all the angular motion of the outward cylinder, we
shall have the motion of the fluid in a quiescent cylinder.

Cor. 5. Therefore if the fluid and outward cylinder are at rest, and the
inward cylinder revolve uniformly, there will be communicated a circular
motion to the fluid, which will be propagated by degrees through the whole
fluid; and will go on continually increasing, till such time as the several
parts of the fluid acquire the motion determined in Cor. 4.

Dor. 6 . And because the fluid endeavours to propagate its motion still

372

THE MATHEMATICAL PRINCIPLES

[Book 11.

farther, its impulse will carry the outmost cylinder also about with it, urn
less the cylinder be violently detained; and accelerate its motion till the
periodic times of both cylinders become equal among themselves. But if
the outward cylinder be violently detained.it will make an effort to retard
the motion of the fluid; and unless the inward cylinder preserve that mo¬
tion by means of some external force impressed thereon, it will make it
oease by degrees.

All these things will be found true by making the experiment in deep
standing water.

PROPOSITION LII. THEOREM XL.

If a solid sphere, in an uniform and infinite fluid, revolves about an axis
given in position with an uniform motion, and the fluid be forced round
by only this impulse of the sphere; and every part of the fluid perse¬
veres uniformly in its motion; I say, that the periodic times of the
parts of the fluid are as the squares of their distances from the centre
of the sphere.

Case 1. Let AFL be a sphere turn¬
ing uniformly about the axis S, and let
the concentric circles BGM, CHN, DIO,
EKP, &c., divide the fluid into innu¬
merable concentric orbs of the same
thickness. Suppose those orbs to be
solid ; and, because the fluid is homo¬
geneous, the impressions which the con¬
tiguous orbs make one upon another
will be (by the supposition) as their
translations from one another, and the
contiguous superficies upon which the
impressions are made. If the impression upon any orb be greater or less
upon its concave than upon its convex side, the more forcible impression
will prevail, and will either accelerate or retard the velocity of the orb, ac¬
cording as it is directed with a conspiring or contrary motion to that of
the orb. Therefore that every orb may persevere uniformly in its motion,
it is necessary that^the impressions made upon both sides of the orb should
be equal, and have contrary directions. Therefore since the impressions
are as the contiguous superficies, and as their translations from one another,
the translations will be inversely as the superficies, that is, inversely as the
squares of the distances of the superficies from the centre. But the differ*
ences of the angular motions about the axis are as those translations applied
to the distances, or as the translations directly and the distances inversely;
that is, by compounding those ratios, as the cubes of the distances inversely.
Therefore if upon the several parts of the infinite right line SABCDEQ

Sec. IX. j

OF NATURAL PHILOSOPHY.

373

there be erected the perpendiculars Aa, B b, C c, Ud, Ee, <fec., reciprocally
proportional to the cubes of SA, SB, SC, SD, SE, &c., the sums of the
differences, that is, the whole angular motions will be as the corresponding
sums of the lines A a, B6, Cc, T>d, Ee, (fee., that is (if to constitute an uni¬
formly fluid medium the number of the orbs be increased and their thick¬
ness diminished in infinitum), as the hyperbolic areas AaQ, B6Q, CcQ,
DdQ,, EeQ,, tfec., analogous to the sums; and the periodic times being re¬
ciprocally proportional to the angular motions, will be also reciprocally
proportional to those areas. Therefore the periodic time of any orb DIO
is reciprocally as the area DdQ,, that is (by the known methods of quadra¬
tures), directly as the square of the distance SD. Which was first to be
demonstrated.

Case 2. From the centre of the sphere let there be drawn a great num¬
ber of indefinite right lines, making given angles with the axis, exceeding
one another by equal differences; and, by these lines revolving about the
axis, conceive the orbs to be cut into innumerable annuli; then will every
annulus have four annuli contiguous to it, that is, one on its inside, one on
its outside, and two on each hand. Now each of these annuli cannot be
impelled equally and with contrary directions by the attrition of the inte¬
rior and exterior annuli, unless the motion be communicated according to
the law which we demonstrated in Case 1. This appears from that dem¬
onstration. And therefore any series of annuli, taken in any right line
extending itself in infinitum from the globe, will move according to the
law of Case 1, except we should imagine it hindered by the attrition of the
annuli on each side of it. But now in a motion, according to this law, no
such is, and therefore cannot be, any obstacle to the motions persevering
according to that law. If annuli at equal distances from the centre
revolve either more swiftly or more slowly near the poles than near the
ecliptic, they will be accelerated if slow, and retarded if swift, by their
mutual attrition; and so the periodic times will continually approach to
equality, according to the law of Case 1. Therefore this attrition will not
at all hinder the motion from going on according to the law of Case l,and
therefore that law will take place; that is, the periodic times of the several
annuli will be as the squares of their distances from the centre of the globe.
Which was to be demonstrated in the second place.

Case 3. Let now every annulus be divided by transverse sections into
innumerable particles constituting a substance absolutely and uniformly
fluid; and because these sections do not at all respect the law of circular
motion, but only serve to produce a fluid substance, the law of circular mo¬
tion will continue the same as before. All the very small annuli will either
not at all change their asperity and force of mutual attrition upon account
of these sections, or else they will change the same equally. Therefore the
proportion of the causes remaining the same, the proportion of the effects

3r4

THE MATHEMATICAL PRINCIPLES

[Book II.

will remain the same also ; that is, the proportion of the motions and thi
periodic times. Q.E.D. But now as the circular motion, and the centri¬
fugal force thence arising, is greater at the ecliptic than at the poles, there
must be some cause operating to retain the several particles in their ciieles;
otherwise the matter that is at the ecliptic will always recede from the
centre, and come round about to the poles by the outside of the vortex,
and from thence return by the axis to the ecliptic with a perpetual circu¬
lation.

Cor. 1. Hence the angular motions of the parts of the fluid about the
axis of the globe are reciprocally as the squares of the distances from the
centre of the globe, and the absolute velocities are reciprocally as the same
squares applied to the distances from the axis.

Cor. 2. If a globe revolve with a uniform motion about an axis of a
given position in a similar and infinite quiescent fluid with an uniform
motion, it will communicate a whirling motion to the fluid like that of a
vortex, and that motion will by degrees be propagated onward in infinitum ;
and this motion will be increased, continually in every part of the fluid, till
the periodical times of the several parts become as the squares of the dis¬
tances from the centre of the globe.

Cor. 3. Because the inward parts of the vortex are by reason of their
greater velocity continually pressing upon and driving forward the external
parts, and by that action are perpetually communicating motion to them,
and at the same time those exterior parts communicate the same quantity
of motion to those that lie still beyond them, and by this action preserve
the quantity of their motion continually unchanged, it is plain that the
motion i3 perpetually transferred from the centre to the circumference of
the vortex, till it is quite swallowed up and lost in the boundless extent of
that circumference. The matter between any two spherical superficies
eoncentrical to the vortex will never be accelerated; because that matter
will be always transferring the motion it receives from the matter nearer
the centre to that matter which lies nearer the circumference.

Cor. 4. Therefore, in order to continue a vortex in the same state of
motion, some active principle is required from which the globe may receive
continually the same quantity of motion which it is always communicating
to the matter of the vortex. Without such a principle it will undoubtedly
come to pass that the globe and the inward parts of the vortex, being al¬
ways propagating their motion to the outward parts, and not receiving any
new motion, will gradually move slower and slower, and at last be carried
round no longer.

Cor. 5. If another globe should be swimming in the same vortex at a
certain distance from its centre, and in the mean time by some force revolve
constantly about an axis of a given inclination, the motion of 'Jiis globe
will drive the fluid round after the manner of a vortex; and at first this

Or NATURAL PHILOSOPHY".

375

Sec. IX.]

new and small vortex will revolve with its globe about the centre of the
other; and in the mean time its motion will creep on farther and farther,
and by degrees be propagated in infinitum , after the manner of the first
vortex. And for the same reason that the globe of the new vortex wait
carried about before by the motion of the other vortex, the globe of this
other will be carried about by the motion of this new vortex, so that the
two globes will revolve about some intermediate point, and by reason of
that circular motion mutually fly from each other, unless some force re¬
strains them. Afterward, if the constantly impressed forces, by which the
globes persevere in their motions, should cease, and every thing be left to
act according to the laws of mechanics, the motion of the globes will lan¬
guish by degrees (for the reason assigned in Cor. 3 and 4), and the vortices
at last will quite stand still.

Cor. 6. If several globes in given places should constantly revolve with
determined velocities about axes given in position, there would arise from
them as many vortices going on in infinitum. For upon the same account
that any one globe propagates its motion in infinitum , each globe apart
will propagate its own motion in infinitum also; so that every part of the
infinite fluid will be agitated with a motion resulting from the actions of
all the globes. Therefore the vortices will not be confined by any certain
limits, but by degrees run mutually into each other; and by the mutual
actions of the vortices on each other, the globes will be perpetually moved
from their places, as was shewn in the last Corollary; neither can they
possibly keep any certain position among themselves, unless some force re¬
strains them. But if those forces, which are constantly impressed upon
the globes to continue these motions, should cease, the matter (for the rea¬
son assigned in Cor. 3 and 4) will gradually stop, and cease to move in
vortices.

Cor. 7. If a similar fluid be inclosed in a spherical vessel, and, by the
uniform rotation of a globe in its centre, is driven round in a vortex; and
the globe and vessel revolve the same way about the same axis, and their
periodical times be as the squares of the semi-diameters; the parts of the
fluid will not go on in their motions without acceleration or retardation
till their periodical times are as the squares of their distances from
the centre of the vortex. No constitution of a vortex can be permanent
but this.

Cor. 8. If the vessel, the inclosed fluid, and the globe, retain this mo¬
tion, and revolve besides with a common angular motion about any given
axis, because the mutual attrition of the parts of the fluid is not changed
by this motion, the motions of the parts among each other will not be
changed; for the translations of the parts among themselves depend upon
this attrition. Any part will persevere in that motion in which its attri-

THE MATHEMATICAL PRINCIPLES

376

[Book II.

tion on one side retards it just as mucli as its attrition on the other side
accelerates it.

Cor. 9. Therefore if the vessel be quiescent, and the motion of the
globe be given, the motion of the fluid will be given. For conceive a plane
to puss through the axis of the globe, and to revolve with a contrary mo¬
tion ; and suppose the sum of the time of this revolution and of the revolu¬
tion of the globe to be to the time of the revolution of the globe as the
square of the semi-diameter of the vessel.to the square of the semi-diameter
of the globe; and the periodic times of the parts of the fluid in respect of
this plane will be as the squares of their distances from the centre of the
globe.

Cor. 10. Therefore if the vessel move about the same axis with the globe,
or with a given velocity about a different one, the motion of the fluid will
be given. For if from the whole system we take away the angular motion
of the vessel, all the motions will remain the same among themselves as
before, by Cor. S, and those motions will be given by Cor. 9.

Cor. 11. If the vessel and the fluid are quiescent, and the globe revolves
with an uniform motion, that motion will be propagated by degrees through
the whole fluid to the vessel, and the vessel will be carried round by it,
unless violently detained; and the fluid and the vessel will be continually
accelerated till their periodic times become equal to the periodic times of
the globe. If the vessel be either withheld by some force, or revolve with
any constant and uniform motion, the medium will come by little and
little to the state of motion defined in Cor. S, 9, 10, nor will it ever perse¬
vere in any other state. But if then the forces, by which the globe and
vessel revolve with certain motions, should cease, and the whole system be
left to act according to the mechanical laws, the vessel and globe, by means
of the intervening fluid, will act upon each other, and will continue to
propagate their motions through the fluid to each other, till their periodic
times become equal among themselves, and the whole system revolves to¬
gether like one solid body.

SCHOLIUM.

In all these reasonings I suppose the fluid to consist of matter of uniform
density and fluidity; I mean, that the fluid is such, that a globe placed
any where therein may propagate with the same motion of its own, at dis¬
tances from itself continually equal, similar and equal motions in the fluid
in the same interval of time. The matter by its circular motion endeavours
to recede from the axis of the vortex, and therefore presses all the matter
that lies beyond. This pressure makes the attrition greater, and the
Separation of the parts more difficult; and by consequence diminishes
the fluidity of the matter. Again ; if the parts of the fluid are in any one
place denser or larger than in the others, the fluidity will be less in that
l lace, because there are fewer superficies where the parts can be separated

%£C- I.X..]

OF NATURAL PHILOSOPHY.

37*7

from each other. In these cases I suppose the defect of the fluidity to be
supplied by the smoothness or softness of the parts, or some other condi¬
tion ; otherwise the matter where it is less fluid will cohere more, and be
more sluggish, and therefore will receive the motion more slowly, and pro¬
pagate it farther than agrees with the ratio above assigned. If the vessel
be not spherical, the particles will move in lines not circular, but answer¬
ing to the figure of the vessel; and the periodic times will be nearly as the
squares of the mean distances from the centre. In the parts between the
centre and the circumference the motions will be slower where the spaces
are wide, and swifter where narrow; but yet the particles will not tend to the
circumference at all the more for their greater swiftness; for they then
describe arcs of less curvity, and the conatus of receding from the centre is
as much diminished by the diminution of this curvature as it is augment¬
ed by the increase of the velocity. As they go out of narrow into wide
spaces, they recede a little farther from the centre, but in doing so are re¬
tarded ; and when they come out of wide into narrow spaces, they are again
accelerated; and so each particle is retarded and accelerated by turns for
ever. These things will come to pass in a rigid vessel; for the state of
vortices in an infinite fluid is known by Cor. 0 of this Proposition.

I have endeavoured in this Proposition to investigate the properties of
vortices, that I might find whether the celestial phenomena can be explain¬
ed by them; for the phenomenon is this, that the periodic times of the
planets revolving about Jupiter are in the sesquiplicate ratio of their dis¬
tances from Jupiter’s centre; and the same rule obtains also among the
planets that revolve about the sun. And these rules obtain also with the
greatest accuracy, as far as has been yet discovered by astronomical obser-
tion. Therefore if those planets are carried round in vortices revolving
about Jupiter and the sun, the vortices must revolve according to that
law. But here we found the periodic times of the parts of the vortex to
be in the duplicate ratio of the distances from the centre of motion; and
this ratio cannot be diminished and reduced to the sesquiplicate, unless
either the matter of the vortex be more fluid the farther it is from the cen¬
tre, or the resistance arising from the want of lubricity in the parts of the
fluid should, as the velocity with which the parts of the fluid are separated
goes on increasing, be augmented with it in a greater ratio than that in
which the velocity increases. But neither of these suppositions seem rea¬
sonable. The more gross and less fluid parts will tend to the circumfer¬
ence, unless they are heavy towards the centre. And though, for the sake
of demonstration, I proposed, at the beginning of this Section, an Hypoth¬
esis that the resistance is proportional to the velocity, nevertheless, it is in
truth probable that the resistance is in a less ratio than that of the velo¬
city ; which granted, the periodic times of the parts of the vortex will be
in a greater than the duplicate ratio of the distances from its centre. If,
as some think, the vortices move more swiftly near the centre, then slower

3~s

THE MATHEMATICAL PRINCIPLES

[Book IT

to a certain limit, then again swifter near the circumference, certainly
neither the sesquiplicate, nor any other certain and determinate ratio, can
obtain in them. Let philosophers then see how that phenomenon of the
sesquiplicate ratio can be accounted for by vortices.

PROPOSITION LIII. THEOREM XLI.

Bodies carried about in a vortex , and returning in the same orb , are of
the same density with the vortex , and are moved according to the
same law with the parts of the vortex , as to velocity and direction oj
motion .

For if any small part of the vortex, whose particles or physical points
preserve a given situation among each other, be supposed to be congealed,
this particle will move according to the same law as before, since no change
is made either in its density, vis insita, or figure. And again; if a congealed
or solid part of the vortex be of the same density with the rest of the vortex,
and be resolved into a fluid, this will move according to the same law as
before, except in so far as its particles, now become fluid, may be moved
among themselves. Neglect, therefore, the motion of the particles among
themselves as not at all concerning the progressive motion of the whole, and
the motion of the whole will be the same as before. But this motion will be
the same with the motion of other parts of the vortex at equal distances
from the centre; because the solid, now resolved into a fluid, is become
perfectly like to the other parts of the vortex. Therefore a solid, if it be
of the same density with the matter of the vortex, will move with the same
motion as the parts thereof, being relatively at rest in the matter that sur¬
rounds it. If it be more dense, it will endeavour more than before to re¬
cede from the centre; and therefore overcoming that force of the vortex,
by which, being, as it were, kept in equilibrio, it was retained in its orbit,
it will recede from the centre, and in its revolution describe a spiral, re¬
turning no longer into the same orbit. And, by the same argument, if it
be more rare, it will approach to the centre. Therefore it can never con¬
tinually go round in the same orbit, unless it be of the same density with
the fluid. But we have shewn in that case that it would revolve accord¬
ing to the same law with those parts of the fluid that are at the same or
equal distances from the centre of the vortex.

Cor. 1. Therefore a solid revolving in a vortex, and continually going
round in the same orbit, is relatively quiescent in the fluid that carries it.

Cor. 2. And if the vortex be of an uniform density, the same body may
revolve at any distance from the centre of the vortex.

SCHOLIUM.

Hence it is manifest that the planets are not carried round in corporeal
vortices; for, according to the Copernican hypothesis, the planets going

V

Sec. IX.] OF NATURAL PHILOSOPHY. 379

round the sun revolve in ellipses, having the sun in their common focus ;
and by radii drawn to the sun describe
areas proportional to the times. But
now the parts of a vortex can never re¬
volve with such a motion. Let AD,

BE, CF, represent three orbits describ¬
ed about the sun S, of which let the
utmost circle CF be concentric to the
sun ; and let the aphelia of the two in¬
nermost be A, B; and their perihelia
D, E. Therefore a body revolving in
the orb CF, describing, by a radius
drawn to the sun, areas proportional to
the times, will move with an uniform motion. And, according to the laws
of astronomy, the body revolving in the orb BE will move slower in its
aphelion B, and swifter in its perihelion E; whereas, according to the
laws of mechanics, the matter of the vortex ought to move more swiftly in
the narrow space between A and C than in the wide space between D and
F; that is, more swiftly in the aphelion than in the perihelion. Now these
two conclusions cohtradict each other. So at the beginning of the sign of
Virgo, where the aphelion of Mars is at present, the distance between the»
orbits of Mars and Yenus is to the distance between the same orbits, at the
beginning of the sign of Pisces, as about 3 to 2; and therefore the matter
of the vortex between those orbits ought to be swifter at the beginning of
Pisces than at the beginning of Virgo in the ratio of 3 to 2 ; for the nar¬
rower the space is through which the same quantity of matter passes in the
same time of one revolution, the greater will be the velocity with which it
passes through it. Therefore if the earth being relatively at rest in this
celestial matter should be carried round by it, and revolve together with it
about the sun, the velocity of the earth at the beginning of Pisces
would be to its velocity at the beginning of Virgo in a sesquialteral ratio.
Therefore the sun’s apparent diurnal motion at the beginning of Virgo
ought to be above 70 minutes, and at the beginning of Pisces less than 48
minutes; whereas, on the contrary, that apparent motion of the sun is
really greater at the beginning of Pisces than at the beginning of Virgo*,
as experience testifies; and therefore the earth is swifter at the beginning
of Virgo than at the beginning of Pisces; so that the hypothesis of vor¬
tices is utterly irreconcileable with astronomical phenomena, and rather
serves to perplex than explain the heavenly motions. How these mo¬
tions are performed in free spaces without vortices, may be understood
by the first Book; and I shall now more fully treat of it in the following
Book.

BOOK III.

In the preceding Books I have laid down the principles of philosophy,
principles not philosophical, but mathematical: such, to wit, as we may
build our reasonings upon in philosophical inquiries. These principles are
the laws and conditions of certain motions, and powers or forces, which
chiefly have respect to philosophy; but, lest they should have appeared of
themselves dry and barren, I have illustrated them here and there with
some philosophical scholiums, giving an account of such things as are of
more general nature, and which philosophy seems chiefly to be founded on ;
such as the density and the resistance of bodies, spaces void of all bodies,
and the motion of light and sounds. It remains that, from the same prin¬
ciples, I now demonstrate the frame of the System of the World. Upon
this subject I had, indeed, composed the third Book in a popular method,
that it might be read by many; but afterward, considering that such as
had not sufficiently entered into the principles could not easily discern the
strength of the consequences, nor lay aside the prejudices to which they had
been many years accustomed, therefore, to prevent the disputes which might
be raised upon such accounts, I chose to reduce the substance of this Book
into the form of Propositions (in the mathematical way), which should be
read by those only who had first made themselves masters of the principles
established in the preceding Books: not that I would advise any one to the
previous study of every Proposition of those Books; for they abound with
such as might cost too much time, even to readers of good mathematical
learning. It is enough if one carefully reads the Definitions, the Laws of
Motion, and the first three Sections of the first Book. He may then pass
on to this Book, and consult such of the remaining Propositions of the
first two Books, as the references in this, and his occasions, shall require.

384

THE MATHEMATICAL PRINCIPLES

[Book IIL

RULES OF REASONING IN PHILOSOPHY.

RULE I.

We are i’o admit no more causes of natural things than such as are both
true and sufficient to explain their appearances.

To this purpose the philosophers say that Nature does nothing in vain,
and more is in vain when less will serve; for Nature is pleased with sim¬
plicity, and affects not the pomp of superfluous causes.

RULE II.

Therefore to the same natural effects we must , as far as possible, assign

the same causes.

As to respiration in a man and in a beast; the descent of stones in Europe
and in America ; the light of our culinary fire and of the sun; the reflec¬
tion of light in the earth, and in the planets.

RULE III.

The qualities of bodies , which admit neither intension nor remission oj
degrees , and ivhich are found to belong to all bodies within the reach
of our experiments , are to be esteemed the universal qualities of all
bodies whatsoever.

For since the qualities of bodies are only known to us by experiments, we
are to hold for universal all such as universally agree with experiments;
and such as are not liable to diminution can never be quite taken away.
We are certainly not to relinquish the evidence of experiments for the sake
of dreams and vain fictions of our own devising; nor are we to recede from
the analogy of Nature, which uses to be simple, and always consonant to
itself. We no other way know the extension of bodies than by our senses,
nor do these reach it in all bodies; but because we perceive extension in
all that are sensible, therefore we ascribe it universally to all others also.
That abundance of bodies are hard, we learn by experience; and because
the hardness of the whole arises from the hardness of the parts, we therefore
justly infer the hardness of the undivided particles not only of the bodies
we feel but of all others. That all bodies are impenetrable, we gather not
from reason, but from sensation. The bodies which we handle we find im¬
penetrable, and thence conclude impenetrability to be an universal property
of all bodies whatsoever. That all bodies are moveable, and endowed with
certain powers (which we call the vires inertice) of persevering in their mo¬
tion, or in their rest, we only infer from the like properties observed in the

OF NATURAL PHILOSOPHY.

385

Book 1II.J

bodies which we have seen. The extension, hardness, impenetrability, mo¬
bility, and vis inertice of the whole, result from the extension, hardness,
impenetrability, mobility, and vires inertice of the parts; and thence we
conclude the least particles of all bodies to be also all extended, and hard
and impenetrable, and moveable, and endowed with their proper vires inertia.
And this is the foundation of all philosophy. Moreover, that the divided
but contiguous particles of bodies may be separated from one another, is
matter of observation; and, in the particles that remain undivided, our
minds are able to distinguish yet lesser parts, as is mathematically demon¬
strated. But whether the parts so distinguished, and not yet divided, may,
by the powers of Nature, be actually divided and separated from one an¬
other, we cannot certainly determine. Yet, had we the proof of but one.
experiment that any undivided particle, in breaking a hard and solid body,
suffered a division, we might by virtue of this rule conclude that the un¬
divided as well as the divided particles may be divided and actually sep¬
arated to infinity.

Lastly, if it universally appears, by experiments and astronomical obser¬
vations, that all bodies about the earth gravitate towards the earth, and
that in proportion to the quantity of matter which they severally contain;
that the moon likewise, according to the quantity of its matter, gravitates
towards the earth; that, on the other hand, our sea gravitates towards the
moon; and all the planets mutually one towards another; and the comets
in like manner towards the sun ; we must, in consequence of this rule, uni¬
versally allow that all bodies whatsoever are endowed with a principle of
mutual gravitation. For the argument from the appearances concludes with
more force for the universal gravitation of all bodies than for their impen¬
etrability ; of which, among those in the celestial regions, we have no ex¬
periments, nor any manner of observation. Not that I affirm gravity to be
essential to bodies: by their vis insita I mean nothing but their vis inertice.
This is immutable. Their gravity is diminished as they recede from the
earth.

RULE IV.

In experimental philosophy we are to look upon propositions collected by
general induction f rom pheenomena as accurately or very nearly true,
notwithstanding any contrary hypotheses that may be imagined , till
such time as other pheenomena occur , by which they may eithei' be made
more accurate , or liable to exceptions .

This rule we must follow, that the argument of induction may not b#
evaded by hypotheses.

25

386

THE MATHEMATICAL PRINCIPLES

[Book III.

PHENOMENA, OR APPEARANCES,

PHENOMENON I.

That the circumjovial planets, by radii drawn to Jupiter’s centre, de¬
scribe areas proportional to the times of description; and that their
periodic times, the fixed stars being- at rest, are in the sesquiplicate
proportion of their distances from its centre.

This we know from astronomical observations. For the orbits of these
planets differ but insensibly from circles concentric to Jupiter; and their
motions in those circles are found to be uniform. And all astronomers
agree that their periodic times are in the sesquiplicate proportion of the
semi-diameters of their orbits; and so it manifestly appears from the fol-
1 owing table.

The periodic times of the satellites of Jupiter.

H 18 h . 27'. 34". 3 d . 13 h . 13' 4.2". 7 d . 3 h . 42' 36". 16 d . 16 h . 32' 9".
The distances of the satellites from Jupiter’s centre.

From the observations of

1

2

3

4

Borelli.

o'i

8|

14

24| 1

,

Townly by the Microm. . .

5,52

8,78

13,47

24,72

semi-diameter of

Cassini by the Telescope .

5

8

13

23 , 1

1 Jupiter.

Cassini by the eclip. of the satel. .

5§

9

14f§

25 t 3 o J

| From the periodic times

5,667

9^017

14,384

25,299

Mr. Pound has determined, by the help of excellent micrometers, the
diameters of Jupiter and the elongation of its satellites after the following
manner. The greatest heliocentric elongation of the fourth satellite from
Jupiter’s centre was taken with a micrometer in a 15 feet telescope, and at
the mean distance of Jupiter from the earth was found about 8' 16". The
elongation of the third satellite was taken with a micrometer in a telescope
of 123 feet, and at the same distance of Jupiter from the earth was found
4' 42". The greatest elongations of the other satellites, at the same dis¬
tance of Jupiter from the earth, are found from the periodic times to be 2'
56" 47"', and V 51" 6'".

The diameter of Jupiter taken with the micrometer in a 123 feet tele¬
scope several times, and reduced to Jupiter’s mean distance from the earth,
proved always less than 40", never less than 38", generally 39". This di¬
ameter in shorter telescopes is 40", or 41"; for Jupiter’s light is a little
dilated by the unequal refrangibility of the rays,' and this dilatation bears
3 less ratio to the diameter of Jupiter in the longer and more perfect tele-
escopes than in those which are shorter and less perfect. The times

OF NATURAL PHILOSOPHY

387

Book. III.]

which two satellites, the first and the third, passed over Jupiter's body, were
observed, from the beginning of the ingress to the beginning of the egress,
and from the complete ingress to the complete egress, with the long tele¬
scope. And from the transit of the first satellite, the diameter of Jupiter
at its mean distance from the earth came forth 3 7\". and from the transit
of the third 37f". There was observed also the time in which the shadow
of the first satellite passed over Jupiter’s body, and thence the diameter of
Jupiter at its mean distance from the earth came out about 37". Let us
suppose its diameter to be 37}" very nearly, and then the greatest elonga¬
tions of the first, second, third, and fourth satellite will be respectively
equal to 5,965, 9,494, 15,141, and 26,63 semi-diameters of Jupiter.

PHENOMENON II.

That the circumsaturnal planets , by radii drawn to Saturn’s centre , de¬
scribe areas proportional to the times of description ; and that their
periodic times, the fixed stars being at rest , are in the sesquiplicate
proportion uf their distances from its centre .

For, as Cassini from his own observations has determined, theii distan¬
ces from Saturn’s centre and their periodic times are as follow.

The periodic times of the satellites of Saturn.
l d . 21 h . 18' 27". 2 d . 1 7 h . 41' 22". 4 d . 12 h . 25' 12". I5 d . 22\ 41' 14".

79 1 . 7\ 48' 00".

The distances of the satellites from Saturn’s centre, in semi diameters oj

its ring .

From observations .1|{|. 2\. 3\. 8. 24

From the periodic times . . . 1,93. 2,47. 3,45. 8. 23,35.

The greatest elongation of the fourth satellite from Saturn’s centre is
commonly determined from the observations to be eight of th >se semi¬
diameters very nearly. But the greatest elongation of this satellite from
Saturn’s centre, when taken with an excellent micrometer in Mr. Huy gens’
telescope of 123 feet, appeared to be eight semi-diameters and T 7 - 0 of a semi¬
diameter. And from this observation and the periodic times the distances
of the satellites from Saturn’s centre in semi-diameters of the ring are 2,1.
2,69. 3,75. 8,7. and 25,35. The diameter of Saturn observed in the same
telescope was found to be to the diameter of the ring as 3 to 7; and the
diameter of the ring, May 28-29, 1719, was found to be 43"; and thence
the diameter of the ring when Saturn is at its mean distance from the
earth is 42", and the diameter of Saturn 18". These things appear so in
very long and excellent telescopes, because in such telescopes the apparent
magnitudes of the heavenly bodies bear a greater proportion to the dilata¬
tion of light in the extremities of those bodies than in shorter telescopes.

3S8 THE MATHEMATICAL PRINCIPLES [BOOK 111

If we, then, reject all the spurious light, the diameter of Saturn will not
amount to more than 16".

PHENOMENON III.

That the five primary planets, Mercury, Venus, Mars, Jupiter, and Sat¬
urn, with their several orbits, encompass the sun.

That Mercury and Venus revolve about the sun, is evident from their
moon-like appearances. When they shine out with a full face, they are, in
respect of us, beyond or above the sun; when they appear half full, they
are about the same height on one side or other of the sun; when horned,
they are below or between us and the sun; and they are sometimes, when
directly under, seen like spots traversing the sun’s disk. That Mars sur¬
rounds the sun, is as plain from its full face when near its conjunction with
the sun, and from the gibbous figure which it shews in its quadratures.
And the same thing is demonstrable of Jupiter and Saturn, from their ap¬
pearing full in all situations; for the shadows of their satellites that appear
sometimes upon their disks make it plain that the light they shine witli is
not their own, but borrowed from the sun.

PHENOMENON IV.

That the fixed stars being at rest, the periodic times of the five primary
planets, and (ivhether of the sun, about the earth, or) of the earth about
the sun, are in the sesquiplicate proportion of their mean distances
from the sun.

This proportion, first observed by Kepler, is now received by all astron¬
omers ; for the periodic times are the same, and the dimensions of the orbits
are the same, whether the sun revolves about the earth, or the earth about
the sun. And as to the measures of the periodic times, all astronomers are
agreed about them. But for the dimensions of the orbits, Kepler and Bul-
lialdus, above all others, have determined them from observations with the
greatest accuracy; and the mean distances corresponding to the periodic
times differ but insensibly from those which they have assigned, and for
the most part fall in between them; as we may see from the following table.

The periodic times with respect to the fixed stars, of the planets and earth
revolving about the sun, in days and decimal parts of a day.

b n * s ? *

10759,275. 4332,514. 686,9785. 365,2565. 224,6176. 87,9692.

The mean distances of the planets and of the earth from the sun .

b ■ V $

According to Kepler . 951000. 519650. 152350.

“ to Bullialdus . 954198. 522520. 152350.

v to the periodic times .... 954006. 520096. 152369

Book III.] of natural philosophy. 389

i ? ¥

According to Kepler . 100000. 72400. 38806

“ to Bullialdus ... . • . . 100000. 72398. 38585

“ to the periodic times. 100000. 72333. 38710.

As to Mercury and Venus, there can be no doubt about their distances
from the sun; for they are determined by the elongations of those planets
from the sun; and for the distances of the superior planets, all dispute is
cut off by the eclipses of the satellites of Jupiter. For by those eclipses
the position of the shadow which Jupiter projects is determined; whence
we have the heliocentric longitude of Jupiter. And from its helio¬
centric and geocentric longitudes compared together, we determine its
distance.

PHENOMENON V.

Then the primary planets , by radii drawn to the earth , describe areas no
wise proportional to the times ; but that the areas which they describe
by radii drawn to the sun are proportional to the times of descrip¬
tion.

For to the earth they appear sometimes direct, sometimes stationary,
nay, and sometimes retrograde. But from the sun they are always seen
direct, and to proceed with a motion nearly uniform, that is to say, a little
swifter in the perihelion and a little slower in the aphelion distances, so as
to maintain an equality in the description of the areas. This a noted
proposition among astronomers, and particularly demonstrable in Jupiter,
from the eclipses of his satellites; by the help of which eclipses, as we have
said, the heliocentric longitudes of that planet, and its distances from the
sun, are determined.

PHENOMENON VI.

That the moon , by a radius drawn to the earttis centre , describes an area
proportional to the time of description.

This we gather from the apparent motion of the moon, compared with
its apparent diameter. It is true that the motion of the moon is a little
disturbed by the action of the sun: but in laying down these Phenomena.
I neglect those *mall and inconsiderable errors.

390

THE MATHEMATICAL PRINCIPLES

[Book III

PROPOSITIONS-

PROPOSITION I. THEOREM I.

That the forces by which the circmnjovial planets are continually drawn
offfrom rectilinear motions, and retained in their proper orbits, tend
to Jupiter's centre ; and are reciprocally as the squares of the distances
of the places of those planets from that centre.

The former part of this Proposition appears from Phmn. I, and Prop.
II or III, Book I; the latter from Pham. I, and Cor. 6, Prop. IV, of the same
Book.

The same thing we are to understand of the planets which encompass
Saturn 3 by Phmn. II.

PROPOSITION II. THEOREM II.

That the forces by which the primary planets are continually drawn off
from rectilinear motions , and retained in their proper orbits, tend to
the sun ; and are reciprocally as the squares of the distances of the
places of those planets from the sun’s centre.

The former part of the Proposition is manifest from Phmn. Y, and
Prop. II, Book I; the latter from Phmn. IV, and Cor. 6, Prop. IV, of the
same Book. But this part of the Proposition is, with great accuracy, de¬
monstrable from the quiescence of the aphelion points; for a very small
aberration from the reciprocal duplicate proportion would (by Cor. 1, Prop.
XLY, Book I) produce a motion of the apsides sensible enough in every
single revolution, and in many of them enormously great.

PROPOSITION III. THEOREM III.

That the force by which the moon is retained in its orbit tends to the
earth ; and is reciprocally as the square of the distance of its place
from the earth’s centre.

The former part of the Proposition is evident from Phmn. Yl, and Prop.
II or III, Book I; the latter from the very slow motion of the moon’s apo¬
gee ; which in every single revolution amounting but to 3° 3' in conse¬
quents, may be neglected. For (by Cor. 1. Prop. XLY, Book I) it ap¬
pears, that, if the distance of the moon from the earth’s centre is to the
semi-diameter of the earth as D to 1, the force, from which such a motion
will result, is reciprocally as D 2 i. e., reciprocally as the power of D,
whose exponent is 2-$^ 5 that 18 t° say, the proportion of the distance
something greater than reciprocally duplicate, bat which comes 59f time?
nearer to the duplicate than to the triplicate proportion. But in regard
that this motion is owing to the action of the sun (as we shall afterwards

Book III.] of natural philosophy. 391

shew), it is here to be neglected. The action of the sun, attracting the
moon from the earth, is nearly as the moon’s distance from the earth; and
therefore (by what we have shewed in Oor. 2, Prop. XLY, Book I) is to the
centripetal force of the moon as 2 to 357,45, or nearly so; that is, as I to
178f £ . And if we neglect so inconsiderable a force of the sun, the re¬
maining force, by which the moon is retained in its orb will be recipro¬
cally as D 2 . This will yet more fully appear from comparing this force
with the force of gravity, as is done in the next Proposition.

Cor. If we augment the mean centripetal force by which the moon is
retained in its orb, first in the proportion of 177f§ to 17Sf£, and then in
the duplicate proportion of the semi-diameter of the earth to the mean dis¬
tance of the centres of the moon and earth, we shall have the centripetal
force of the moon at the surface of the earth; supposing this force, in de¬
scending to the earth’s surface, continually to increase in the reciprocal
duplicate proportion of the height.

PROPOSITION IV. THEOREM IV.

That the moon gravitates towards the earth, and by the jorce oj gravity

is continually drawn off from a rectilinear motion, and retained in

its orbit.

The mean distance of the moon from the earth in the syzygies in semi¬
diameters of the earth, is, according to Ptolemy and most astronomers,
59 : according to Vendelin and Huy gens, 60 ; to Copernicus, 601 • to
Street, 60f; and to Tycho, 56|. But Tycho, and all that follow his ta¬
bles of refraction, making the refractions of the sun and moon (altogether
against the nature of light) to exceed the refractions of the fixed stars, and
that by four or five minutes near the horizon, did thereby increase the
moon's horizontal parallax by a like number of minutes, that is, by a
twelfth or fifteenth part of the whole parallax. Correct this error, and
the distance will become about 60 i semi-diameters of the earth, near to
what others have assigned. Let us assume the mean distance of 60 diam¬
eters in the syzygies; and suppose one revolution of the moon, in respect
of the fixed stars, to be completed in 27 d . 7 h . 43', as astronomers have de¬
termined ; and the circumference of the earth to amount to 123249600
Paris feet, as the French have found by mensuration. And now if we
imagine the moon, deprived of all motion, to be let go, so as to descend
towards the earth with the impulse of all that force by which (by Cor.
Prop. Ill) it is retained in its orb, it will in the space of one minute of time,
describe in its fall 15^ Paris feet. This we gather by a calculus, founded
either upon Prop. XXXVI, Book [, or (which comes to the same thing)
upon Cor. 9, Prop. IV, of the same Book. For the versed sine of that arc,
which the moon, in the space of one minute of time, would by its mean

392

THE MATHEMATICAL PRINCIPLES

[Book III

motion describe at the distance of 60 semi-diameters of the earth, is nearly
15 T V Paris feet, or more accurately 15 feet, 1 inch, and 1 line £. Where¬
fore, since that force, in approaching to the earth, increases in the recipro¬
cal duplicate proportion of the distance, and, upon that account, at the
surface of the earth, is 60 X 60 times greater than at the moon, a body
in our regions, falling with that force, ought in the space of one minute of
time, to describe 60 X 60 X 15 T \- Paris feet; and, in the space of one sec¬
ond of time, to describe 15^ of those feet; or more accurately 15 feet, 1
inch, and 1 line f. And with this very force we actually find that bodies
here upon earth do really descend : for a pendulum oscillating seconds in
the latitude of Paris will be 3 Paris feet, and 8 lines \ in length, as Mr.
Huygens has observed. And the space which a heavy body describes
by falling in one second of time is to half the length of this pendulum in
the duplicate ratio of the circumference of a circle to its diameter (as Mr.
Huygens has also shewn), and is therefore 15 Paris feet, 1 inch, 1 line J.
And therefore the force by which the moon is retained in its orbit becomes,
at the very surface of the earth, equal to the force of gravity which we ob¬
serve in heavy bodies there. And therefore (by Rule I and II) the force by
which the moon is retained in its orbit is that very same force which we
commonly call gravity ; for, were gravity another force different from that,
then bodies descending to the earth with the joint impulse of both forces
would fall with a double velocity, and in the space of one second of time
would describe 30^ Paris feet; altogether against experience.

This calculus is founded on the hypothesis of the earth’s standing still ;
for if both earth and moon move about the sun, and at the same time about
their common centre of gravity, the distance of the centres of the moon and
earth from one another will be 60^ semi-diameters of the earth; as may
be found by a computation from Prop. LX, Book I.

SCHOLIUM.

The demonstration of this Proposition may be more diffusely explained
after the following manner. Suppose several moons to revolve about the
earth, as in the system of Jupiter or Saturn; the periodic times of these
moons (by the argument of induction) would observe the same law which
Kepler found to obtain among the planets; and therefore their centripetal
forces would be reciprocally as the squares of the distances from the centre
of the earth, by Prop. I, of this Book. Now if the lowest of these were
very small, and were so near the earth as almost to touch the tops of the
highest mountains, the centripetal force thereof, retaining it in its orb,
would be very nearly equal to the weights of any terrestrial bodies that
should be found upon the tops of those mountains, as may be known by
the foregoing computation. Therefore if the same little moon should be
deserted by its centrifugal force that carries it through its orb, and so he

OF NATURAL PHILOSOPHY.

393

Book III.]

iisabled from going onward therein, it would descend to the earth; and
that with the same velocity as heavy bodies do actually fall with upon the
tops of those very mountains; because of the equality of the forces that
oblige them both to descend. And if the force by which that lowest moon
would descend were different from gravity, and if that moon were to gravi¬
tate towards the earth, as we find terrestrial bodies do upon the tops of
mountains, it would then descend with twice the velocity, as being impel¬
led by both these forces conspiring together. Therefore since both these
forces, that is, the gravity of heavy bodies, and the centripetal forces of the
moons, respect the centre of the earth, and are similar and equal between
themselves, they will (by Rule I and II) have one and the same cause. And
therefore the force which retains the moon in its orbit is that very force
which we commonly call gravity ; because otherwise this little moon at the
top of a mountain must either be without gravity, or fall twice as swiftly
as heavy bodies are wont to do.

PROPOSITION V. THEOREM V.

'Vhat the circumjovial planets gravitate toicards Jupiter ; the circnnisal-
urnal towards Saturn; the circumsolar toicards the sun ; and by the
forces of their gravity are drawn off from rectilinear motions , and re¬
tained in curvilinear orbits.

For the revolutions of the circumjovial planets about Jupiter, of the
circumsaturnal about Saturn, and of Mercury and Venus, and the other
circumsolar planets, about the sun, are appearances of the same sort with
the revolution of the moon about the earth; and therefore, by Rule II,
must be owing to the same sort of causes; especially since it has been
demonstrated, that the forces upon which those revolutions depend tend to
the centres of Jupiter, of Saturn, and of the sun; and that those forces, in
receding from Jupiter, from Saturn, and from the sun, decrease in the same
proportion, and according to the same law, as the force of gravity does in
receding; from the earth.

Cor. 1. There is, therefore, a power of gravity tending to all the plan¬
ets; for, doubtless, Venus, Mercury, and the rest, are bodies of the same
sort with Jupiter and Saturn. And since all attraction (by Law III) is
mutual, Jupiter will therefore gravitate towards all his own satellites, Sat¬
urn towards his, the earth towards the moon, and the sun towards all the
primary planets.

Cor. 2. The force of gravity which tends to any one planet is re¬
ciprocally as the square of the distance of places from that planet’s
centre.

Cor. 3. All the planets do mutually gravitate towards one another, by
Cor. I and 2. And hence it is that Jupiter and Saturn, when near theii

394

THE MATHEMATICAL PRINCIPLES

[Book III

conjunction, by their mutual attractions sensibly disturb each other’s mo¬
tions. So the sun disturbs the motions of the moon; and both sun ini
moon disturb our sea, as we shall hereafter explain.

SCHOLIUM.

The force which retains the celestial bodi s in their orbits has been
hitherto called centripetal force; but it being now made plain that it can
be no other than a gravitating force, we shall hereafter call it gravity.
For the cause of that centripetal force which retains the moon in its orbit
will extend itself to all the planets, by Rule I, II, and IV.

PROPOSITION YI. THEOREM YI.

That all bodies gravitate towards every planet; and that the weights of
bodies towards any the same planet, at equal distances from the centre
of the planet , are proportional to the quantities of matter which they
severally contain.

It has been, now of a long time, observed by others, that all sorts of
heavy bodies (allowance being made for the inequality of retardation which
they suffer from a small power of resistance in the air) descend to the
earth from equal heights in equal times; and that equality of times we
may distinguish to a great accuracy, by the help of pendulums. I tried the
thing in gold, silver, lead, glass, sand, commpn salt, wood, water, and wheat.
I provided two wooden boxes, round and equal: I filled the one with wood,
and suspended an equal weight of gold (as exactly as I could) in the centre
of oscillation of the other. The boxes hanging by equal threads of 11 feet
made a couple of pendulums perfectly equal in weight and figure, and
equally receiving the resistance of the air. And, placing the one by the
other, I observed them to play together forward and backward, for a long
time, wi h equal vibrations. And therefore the quantity of matte* : n the
gold (by Cor. 1 and (i, Prop. XXIV, Book II) "was to the quantity ot mat¬
ter in the wood as the action of the motive force (or vis matrix) upon all
the gold to the action of the same upon all the wood ; that is, as the weight
of the one to the weight of the other: and the like happened in the other
bodies. By these experiments, in bodies of the same weight, 1 could man¬
ifestly have discovered a difference of matter less than the thousandth part
of the whole, had any such been. But, without all doubt, the nature of
gravity towards the planets is the same as towards the earth. For, should
we imagine our terrestrial bodies removed to the orb of the moon, and
there, together with the moon, deprived of all motion, to be let go, so as to
fall together towards the earth, it is certain, from what we have demonstra¬
ted before, that, in equal times, they would describe equal spaces with the
moon, and of consequence are to the moon, in quantity of matter, as their
weights to its weight. Moreover, since the satellites of Jupiter perform

OF NATURAL PHILOSOPHY.

395

Book III.]

their revolutions in times which observe the sesquipluate pr portion ol
their distances from Jupiter’s centre, their accelerative gravities towards
Jupiter will be reciprocally as the squares of their distances from Jupiter’s
centre; that is, equal, at equal distances. And, therefore, these satellites,
if supposed to fall towards Jupiter from equal heights, would describe equal
spaces in equal times, in like manner as heavy bodies do on our earth.
And, by the same argument, if the circumsolar planets were supposed to be
let fall at equal distances from the sun, they would, in their descent towards
the sun, describe equal spaces in equal times. But forces which equally
accelerate unequal bodies must be as those bodies: that is to say, the weights
;f the planets towards the sun, must be as their quantities of matter,
further, that the weights of Jupiter and of his satellites towards the sun
are proportional to the several quantities of their matter, appears from the
exceedingly regular motions of the satellites (by Cor. 3, Prop. LXY, Book
I). For if some of those bodies were more strongly attracted to the sun in
proportion to their quantity of matter than others, the motions of the sat¬
ellites would be disturbed by that inequality of attraction (by Cor. 2, Prop.
LXV, Book I). If, at equal distances from the sun, any satellite, in pro¬
portion to the quantity of its matter, did gravitate towards the sun with a
force greater than Jupiter in proportion to his, according to any given pro¬
portion, suppose of d to e; then the distance between the centres of the sun
and of the satellite’s orbit would be always greater than the distance be¬
tween the centres of the sun and of Jupiter nearly in the subduplicate of
that proportion: as by some computations I have found. And if the sat¬
ellite did gravitate towards the sun with a force, lesser in the proportion of e
to d, the distance of the centre of the satellite’s orb from the sun would be
less than the distance of the centre of Jupiter from the sun in the subdu¬
plicate of the same proportion. Therefore if, at equal distances from the
sun, the accelerative gravity of any satellite towards the sun were greater
or less than the accelerative gravity of Jupiter towards the sun but by one T oVo
part of the whole gravity, the distance of the centre of the satellite’s orbit
from the sun would be greater or less than the distance of Jupiter from the
sun by one 2 oVo P ar ^ the whole distance; that is, by a nf h part of the
distance of the utmost satellite from the centre of Jupiter; an eccentricity
of the orbit which would be very sensible. But the orbits of the satellites
are concentric to Jupiter, and therefore the accelerative gravities of Jupiter,
and of all its satellites towards the sun, are equal among themselves. And
by the same argument, the weights of Saturn and of his satellites towards
the sun, at equal distances from the sun, are as their several quantities of
matter; and the weights of the moon and of the earth towards the sun are
either none, or accurately proportional to the masses of matter which they
contain. But some they are, by Cor. 1 and 3, Prop. Y.

But further; the weights of all the parts of every planet t jwards any other

396

THE MATHEMATICAL PRINCIPLES

[Book II]

planet are one to another as the matter in the several parts; for if some
parts did gravitate more, others less, than for the quantity of their matter,
then the whole planet, according to the sort of parts with which it most
abounds, would gravitate more or less than in proportion to the quantity of
matter in the whole. Nor is it of any moment whether these parts are
external or internal; for if, for example, we should imagine the terrestrial
bodies with us to be raised up to the orb of the moon, to be there compared
with its body : if the weights of such bodies were to the weights of the ex¬
ternal parts of the moon as the quantities of matter in the one and in the
other respectively ; but to the weights of the internal parts in a greater or
less proportion, then likewise the weights of those bodies would be to the
weight of the whole moon in a greater or less proportion; against what
we have shewed above.

Cor. 1. Hence the weights of bodies do not depend upon their forms
and textures; for if the weights could be altered with the forms, they
would be greater or less, according to the variety of forms, in equal matter;
altogether against experience.

Cor. 2. Universally, all bodies about the earth gravitate towards the
earth; and the weights of all, at equal distances from the earth’s centre,
are as the quantities of matter which they severally contain. This is the
quality of all bodies within the reach of our experiments ; and therefore
(by Rule III) to be affirmed of all bodies whatsoever. If the ccther , or anj
other body, were either altogether void of gravity, or were to gravitate lesf
in proportion to its quantity of matter, then, because (according to Aris¬
totle, Des Cartes , and others) there is no difference betwixt that and other
bodies but in mere form of matter, by a successive change from form to
form, it might be changed at last into a body of the same condition with
those wffiich gravitate most in proportion to their quantity of matter; and,
on the other hand, the heaviest bodies, acquiring the first form of that
body, might by degrees quite lose their gravity. And therefore the weights
would depend upon the forms of bodies, and with those forms might be
changed: contrary to what was proved in the preceding Corollary.

Cor. 3. All spaces are not equally full; for if all spaces were equally'
full, then the specific gravity of the fluid which fills the region of the air,
on account of the extreme density of the matter, would fall nothing short
of the specific gravity of quicksilver, or gold, or any other the most dense
body; and, therefore, neither gold, nor any other body, could descend in
air ; for bodies do not descend in fluids, unless they are specifically heavier
than the fluids. And if the quantity of matter in a given space can, by
any rarefaction, be diminished, what should hinder a diminution to
infinity ?

Cor. 4. If all the solid particles of all bodies are of the same density,
nor can be rarefied without pores, a void, space, or vacuum must be granted

OF NATURAL PHILOSOPHY.

397

Book III.]

Bj bodies of the same density, I mean those whose vires inertia, are in the
proportion of their bulks.

Cor. 5. The power of gravity is of a different nature from the power ol
magnetism ; for the magnetic attraction is not as the matter attracted.
Some bodies are attracted more by the magnet; others less; most bodies
not at all. The power of magnetism in one and the same body may be
increased and diminished; and is sometimes far stronger, for the quantity
of matter, than the power of gravity j and in receding from the magnet
decreases not in the duplicate but almost in the triplicate proportion of the
distance, as nearly as I could judge from some rude observations.

PROPOSITION VII. THEOREM VII.

That there is a power of gravity tending to all bodies , proportional to

the several quantities of matter which they contain .

That all the planets mutually gravitate one towards another, we have
proved before; as well as that the force of gravity towards every one of them,
considered apart, is reciprocally as the square of the distance of places from
the centre of the planet. And thence (by Prop. LXIX, Book I, and its
Corollaries) it follows, that the gravity tending towards all the planets is
proportional to the matter which they contain.

Moreover, since all the parts of any planet A gravitate towards any
other planet B ; and the gravity of every part is to the gravity of the
whole as the matter of the part to the matter of the whole; and (by Law
III) to every action corresponds an equal re-action ; therefore the planet B
will, on the other hand, gravitate towards all the parts of the planet A;
and its gravity towards any one part will be to the gravity towards the
whole as the matter of the part to the matter of the whole. Q.E.D.

Cor. 1. Therefore the force of gravity towards any whole planet arises
from, and is compounded of, the forces of gravity towards all its parts.
Magnetic and electric attractions afford us examples of this; for all at¬
traction towards the whole arises from the attractions towards the several
parts. The thing may be easily understood in gravity, if we consider a
greater planet, as formed of a number of lesser planets, meeting together in
one globe; for hence it would appear that the force of the whole must
arise from the forces of the component parts. If it is objected, that, ac¬
cording to this law, all bodies with us must mutually gravitate one to¬
wards another, whereas no such gravitation any where appears, I answer,
that since the gravitation towards these bodies is to the gravitation towards
the whole earth as these bodies are to the whole earth, the gravitation to¬
wards them must be far less than to fall under the observation of our senses.

Cor. 2. The force of gravity towards the several equal particles of any
body is reciprocally as the square of the distance of places from the parti¬
cles ; as appears from Cor. 3, Prop. LXXIV, Book I.

39S

THE MATHEMATICAL PRINCIPLES

[Book HI

PROPOSITION VIII. THEOREM VIII.

Tn two spheres mutually gravitating each towards the other , if the matter
in places on all sides round about and equi-distant from the centres is
similar , the weight of either sphere towards the other will be recipro¬
cally as the square of the distance betiveen their centres.

After I had found that the force of gravity towards a whole planet did
arise from and -was compounded of the forces of gravity towards all its
parts, and towards every one part was in the reciprocal proportion of the
squares of the distances from the part, I w r as yet in doubt whether that re¬
ciprocal duplicate proportion did accurately hold, or but nearly so, in the
total force compounded of so many partial ones; for it might be that the
proportion which accurately enough took place in greater distances should
be wide of the truth near the surface of the planet, where the distances of
the particles are unequal, and their situation dissimilar. But by the help
of Prop. LXXV and LXXVI, Book I, and their Corollaries, I was at last
satisfied of the truth of the Proposition, as it now lies before us.

Cor. 1. Hence we may find and compare together the weights of bodies
towards different planets; for the weights of bodies revolving in circles
about planets are (by Cor. 2, Prop. IV, Book I) as the diameters of the
circles directly, and the squares of their periodic times reciprocally; and
their weights at the surfaces of the planets, or at any other distances from
their centres, are (by this Prop.) greater or less in the reciprocal duplicate
proportion of the distances. Thus from the periodic times of Venus, re¬
volving about the sun, in 224' 1 . 16f h , of the utmost circumjovial satellite
revolving about Jupiter, in 16'. 10 -?y h .; of the Huygenian satellite about
Saturn in 15 d . 22| h .; and of the moon about the earth in 27 (l . 7 h . 43';
compared with the mean distance of Venus from the sun, and with the
greatest heliocentric elongations of the outmost circumjovial satellite
from Jupiter’s centre, S' 16"; of the Huygenian satellite from the centre
of Saturn, 3' 4" ; and of the moon from the earth, 10' 33": by computa¬
tion I found that the weight of equal bodies, at equal distances from the
centres of the sun, of Jupiter, of Saturn, and of the earth, towards the sun,
Jupiter, Saturn, and the earth, were one to another, as 1, T oVt> s oVt? an ^
respectively. Then because as the distances are increased or di¬
minished, the weights are diminished or increased in a duplicate ratio, the
weights of equal bodies towards the sun, Jupiter, Saturn, and the earth,
at the distances 10000,997,791, and 109 from their centres, that is, at their
very superficies, will be as 10000, 943, 529, and 435 respectively. How
much the weights of bodies are at the superficies of the moon, will be
shewn hereafter.

Cor. 2. Hence likewise we discover the quantity of matter in the several

OF NATURAL PHILOSOPHY.

Hook III.J

planets; for their quantities of matter are as the forces of gravity at equai
distances from their centres; that is, in the sun, Jupiter, Saturn, and the
earth, as 1, T oV?> toVtj an d tf bVs 2 respectively. If the parallax of the
sun be taken greater or less than 10" 30'", the quantity of matter in
the earth must be augmented or diminished in the triplicate of that pro¬
portion.

Cor. 3. Hence also we tind the densities of the planets; for (by Prop.
LXXII, Book l) the weights of equal and similar bodies towards similar
spheres are, at the surfaces of those spheres, as the diameters of the spheres •
and therefore the densities of dissimilar spheres are as those weights applied
to the diameters of the spheres. But the true diameters of the Sun, Jupi¬
ter, Saturn, and the earth, were one to another as 10000, 997, 791, and
109; and the weights towards the same as 10000, 943, 529, and 435 re¬
spectively ; and therefore their densities are as 100, 94|, 67, and 400. The
density of the earth, which comes out by this computation, does not depend
upon the parallax of the sun, but is determined by the parallax of the
moon, and therefore is here truly defined. The sun, therefore, is a little
denser than Jupiter, and Jupiter than Saturn, and the earth four times
denser than the sun; for the sun, by its great heat, is kept in a sort of
a rarefied state. The moon is denser than the earth, a3 shall appear after¬
ward.

Cor. 4. The smaller the planets are, they are, cceteris paribus , of so
much the greater density; for so the powers of gravity on their several
surfaces come nearer to equality. They are likewise, cceteris paribus, of
the greater density, as they are nearer to the sun. So Jupiter is more
dense than Saturn, and the earth than Jupiter; for the planets were to be
placed at different distances from the sun, that, according to their degrees
of density, they might enjoy a greater or less proportion to the sun’s heat.
Our water, if it were removed as far as the orb of Saturn, would be con¬
verted into ice, and in the orb of Mercury would quickly fly away in va¬
pour ; for the light of the sun, to which its heat is proportional, is seven
times denser in the orb of Mercury than with us: and by the thermometer
I have found that a sevenfold heat of our summer sun will make water
boil. Nor are we to doubt that the matter of Mercury is adapted to its
heat, and is therefore more dense than the matter of our earth; since, in a
denser matter, the operations of Nature require a stronger heat.

PROPOSITION IX. THEOREM IX.

That the force of gravity , considered downward from the surjace

of the planets, decreases nearly in the proportion of the distances from

their centres.

If the matter of the planet were of an uniform density, this Proposi¬
tion would be accurately true (by Prop. LXXIII. Book I). The error,

THE MATHEMATICAL PRINCIPLES

400

[Book I1L

therefore, can be no greater than what may arise from the inequality of
the density.

PROPOSITION X. THEOREM X.

That the mot ions of the planets in the heavens may subsist an exceedingly

long time.

In the Scholium of Prop. XL, Book II, I have shewed that a globe ol
water lrozen into ice, and moving freely in our air, in the time that it would
describe the length of its semi-diameter, would lose by the resistance of the
air 45 V 6 P art °f its motion; and the same proportion holds nearly in all
globes, how great soever, and moved with whatever velocity. But that our
globe of earth is of greater density than it would be if the whole
consisted of water only, I thus make out. If the whole consisted of
water only, whatever was of less density than water, because of its >ess
specific gravity, would emerge and float above. And upon this account, if
a globe of terrestrial matter, covered on all sides with water, was less dense
than water, it would emerge somewhere; and, the subsiding water falling-
back. would be gathered to the opposite side. And such is the condition
of our earth, which in a great measure is covered with seas. The earth, if
it was not for its greater density, would emerge from the seas, and, accord¬
ing to its degree of levity, would be raised more or less above their surface,
the water of the seas flowing backward to the opposite side. By the same
argument, the spots of the sun, which float upon the lucid matter thereof,
are lighter than that matter; and, however the planets have been formed
while they were yet in fluid masses, all the heavier matter subsided to the
centre. Since, therefore, the common matter of our earth on the surface
thereof is about twice as heavy as water, and a little lower, in mines, is
found about three, or four, or even five times more heavy, it is probable that
the quantity of the whole matter of the earth may be five or six times
greater than if it consisted all of water ; especially since I have before
shewed that the earth is about four times more dense than Jupiter. If,
therefore, Jupiter is a little more dense than water, in the space of thirty
days, in which that planet describes the length of 459 of its semi-diame¬
ters, it would, in a medium of the same density with our air, lose almost a
tenth part of its motion. But since the resistance of mediums decreases
in proportion to their weight or density, so that water, which is 13f times
lighter than quicksilver, resists less in that proportion; and air, which is
860 times lighter than water, resists less in the same proportion ; therefore
in the heavens, where the weight of the medium in which the planets move
is immensely diminished, the resistance will almost vanish.

It is shewn in the Scholium of Prop. XXII, Book II, that at the height
of 200 miles above the earth the air is more rare than it is at the super¬
ficies of the earth in the ratio of 30 to 0,0000000000003998, or as

OF NATURAL PHILOSOPHY.

401

Book III.]

75000000000000 to 1 nearly. And hence the planet Jupiter, revolving in
a medium of the same density with that superior air, would not lose by the
resistance of the medium the 1000000th part of its motion in 1000000
years. In the spaces near the earth the resistance is produced only by the
air, exhalations, and vapours. When these are carefully exhausted by the
air-pump from under the receiver, heavy bodies fall within the receiver with
perfect freedom, and without the least sensible resistance: gold itself, and
the lightest down, let fall together, will descend with equal velocity; and
though they fall through a space of four, six, and eight feet, they will come
to the bottom at the same time; as appears from experiments. And there¬
fore the celestial regions being perfectly void of air and exhalations, the
planets and comets meeting no sensible resistance in those 3paces will con¬
tinue their motions through them for an immense tract of time.

HYPOTHESIS I.

That the centre of the system of the 'world is immovable.

This is acknowledged by all, while some contend that the earth,
others that the sun, is fixed in that centre. Let us see what may from
hence follow.

PROPOSITION XI. THEOREM XI.

That the common centre of gravity of the earth , the su?i } and all the
planets , is immovable.

For (by Cor. 4 of the Laws) that centre either is at rest, or moves uni¬
formly forward in a right line; but if that centre moved, the centre of the
world would move also, against the Hypothesis.

PROPOSITION XII. THEOREM XII.

That the sun is agitated by a perpetual motion , but never recedes far
from the common centre of gravity of all the planets.

For since (by Cor. 2, Prop. VIII) the quantity of matter in the sun is to
the quantity of matter in Jupiter as 1067 to 1; and the distance of Jupi¬
ter from the sun is to the semi-diameter of the sun in a proportion but a
small matter greater, the common centre of gravity of Jupiter and the sun
will fall upon a point a little without the surface of the sun. By the same
argument, since the quantity of matter in the sun is to the quantity of
matter in Saturn as 3021 to 1, and the distance of Saturn from the sun is
to the semi-diameter of the sun in a proportion but a small matter less,
the common centre of gravity of Saturn and the sun will fall upon a point
a little within the surface of the sun. And, pursuing the principles of this
computation, we should find that though the earth and all the planets were
placed on one side of the sun, the distance of the common centre of gravity
of all from the centre of the sun would scarcely amount to one diameter of

26

102

THE MATHEMATICAL PRINCIPLES

[Book III.

the sun. In other cases, the distances of those centres are always less; and
therefore, since that centre of gravity is in perpetual rest, the sun, accord¬
ing to the various positions of the planets, must perpetually be moved every
way, but will never recede far from that centre.

Con. Hence the common centre of gravity of the earth, the sun, and all
the planets, is to lie esteemed the centre of the world; for since the earth,
the sun, and all the planets, mutually gravitate one towards another, and
are therefore, according to their powers of gravity, in perpetual agitation,
as the Laws of Motion require, it is plain that their moveable centres can¬
not he taken for the immovable centre of the world. If that body were to
be placed in the centre, towards which other bodies gravitate most (accord¬
ing to common opinion), that privilege ought to be allowed to the sun; but
since the sun itself is moved, a fixed point is to be chosen from which the
centre of the sun recedes least, and from which it would recede yet
less if the body of the sun were denser and greater, and therefore less apt
to be moved.

PROPOSITION XIII. THEOREM XIII.

The planets move in ellipses which have their common focus in the centre
of the sun ; and , by radii drawn, to that centre , they describe areas pro¬
portional to the times of description.

We have discoursed above of these motions from the Phenomena. Now
that we know the principles on which they depend, from those principles
we deduce the motions of the heavens d priori. Because the weights of
the planets towards the sun are reciprocally as the squares of their distan¬
ces from the sun’s centre, if the sun was at rest, and the other planets did
not mutually act one upon another, their orbits would be ellipses, having
the sun in their common focus; and they would describe areas proportional
to the times of description , by Prop. I and XI, and Cor. 1, Prop. XIII,
Book I. But the mutual actions of the planets one upon another are so
very small, that they may be neglected; and by Prop. LXVI, Book I, they
less disturb the motions of the planets around the sun in motion than if
those motions were performed about the sun at rest.

It is true, that the action of Jupiter upon Saturn is not to be neglected:
for the force of gravity towards Jupiter is to the force of gravity towards
the sun (at equal distances, Cor. 2, Prop. VIII) as 1 to 1067; and therefore
in the conjunction of Jupiter and Saturn, because the distance of Saturn
from Jupiter is to the distance of Saturn from the sun almost as 4 to 9, the
gravity of Saturn towards Jupiter will be to the gravity of Saturn towards
the sun as 81 to 16 X 1067; or, as 1 to about 211. And hence arises a
perturbation of the orb of Saturn in every conjunction of this planet with
Tupiter, so sensible, that astronomers are puzzled with it. As the planet

OF NATURAL PHILOSOPHY.

403

Book III.]

is differently situated in these conjunctions, its eccentricity is sometimes
augmented, sometimes diminished; its aphelion is sometimes carried for¬
ward, sometimes backward, and its mean motion is by turns accelerated and
retarded; yet the whole error in its motion about the sun, though arising
from so great a force, may be almost avoided (except in the mean motion)
by placing the lower focus of its orbit in the common centre of gravity of
Jupiter and the sun (according to Prop. LXVII, Book I), and therefore that
error, when it is greatest, scarcely exceeds two minutes; and the greatest
error in the mean motion scarcely exceeds two minutes yearly. But in the
conjunction of Jupiter and Saturn, the accelerative forces of gravity of the
sun towards Saturn, of Jupiter towards Saturn, and of Jupiter towards the

sun, are almost as 16, 81, and — —? or 166609 ; and therefore

the difference of the forces of gravity of the sun towards Saturn, and of
Jupiter towards Saturn, is to the force of gravity of Jupiter towards the
sun as 65 to 156609, or as 1 to 2409. But the greatest power of Saturn
to disturb the motion of Jupiter is proportional to this difference; and
therefore the perturbation of the orbit of Jupiter is much less than that of
Saturn’s. The perturbations of the other orbits are yet far less, except that
the orbit of the earth is sensibly disturbed by the moon. The common
centre of gravity of the earth and moon moves in an ellipsis about the sun
in the focus thereof, and, by a radius drawn to the sun, describes areas pro¬
portional to the times of description. But the earth in the mean time by
a menstrual motion is revolved about this common centre.

PROPOSITION XIV. THEOREM XIV.

The aphelions and nodes of the orbits of the planets are fixed .

The aphelions are immovable by Prop. XI, Book I; and so are the
planes of the orbits, by Prop. I of the same Book. And if the planes are
fixed, the nodes must be so too. It is true, that some inequalities may
arise from the mutual actions of the planets and comets in their revolu¬
tions ; but these will be so small, that they may be here passed by.

Cor. 1. The fixed stars are immovable, seeing they keep the same posi¬
tion to the aphelions and nodes of the planets.

Cor. 2. And since these stars are liable to no sensible parallax from the
annual motion of the earth, they can have no force, because of their im¬
mense distance, to produce any sensible effect in our system. Not to
mention that the fixed stars, every where promiscuously dispersed in the
heavens, by their contrary attractions destroy their mutual actions, by
Prop. LXX, Book I.

SCHOLIUM.

Since the planets near the sun (viz. Mercury, Venus, the Earth, and

104

THE MATHEMATICAL PRINCIPLES

[B'.OK III

Mars) are so small that they can act with hut little force upon each other,
therefore their aphelions and nodes must be fixed, excepting in so far as
they are disturbed by the actions of Jupiter and Saturn, and other higher
bodies. And hence we may find, by the theory of gravity, that their aphe¬
lions move a little in consequeniia, in respect of the fixed stars, and that
in the sesquiplicate proportion of their several distances from the sun. So
that if the aphelion of Mars, in the space of a hundred years, is carried
33' 20" in consequentia, in respect of the fixed stars ; the aphelions of the
Earth, of Venus, and of Mercury, will in a hundred years be carried for¬
wards 1 7' 40"; 10' 53 ', and 4' 16", respectively. But these motions are
so inconsiderable, that we have neglected them in this Proposition,

PROPOSITION XV. PROBLEM I.

To find the principal diameters uf the orbits of the planets.

They are to be taken in the sub-sesquiplicate proportion of the periodic
times, by Prop. XV, Book I, and then to be severally augmented in the
proportion of the sum of the masses of matter in the sun and each planet
to the first of two mean proportionals betwixt that sum and the quantity of
matter in the sun, by Prop. LX, Book I.

PROPOSITION XVI. PROBLEM II.

To find the eccentricities and aphelions of the planets.

This Problem is resolved by Prop. XVIII, Book I.

PROPOSITION XVII. THEOREM XV.

That the diurnal motions of the planets are uniform , and that the
libration of the moon arises from its diurnal motion.

The Proposition is proved from the first Law of Motion, and Cor. 22,
Prop. LXVI, Book I. Jupiter, with respect to the fixed stars, revolves in
O' 1 . 56 ; Mars in 24 h . 39'; Venus in about 23 h .; the Earth in 23' 1 . 56'; the
Sun in 25-*- days, and the moon in 27 days, 7 hours, 43'. These things
appear by the Phenomena. The spots in the sun’s body return to the
same situation on the sun’s disk, with respect to the earth, in 27} days; and
therefore with respect to the fixed stars the sun revolves in about 25} days.
But because the lunar day, arising from its uniform revolution about its
axis, is menstrual, that is, equal to the time of its periodic revolution in
its orb , therefore the same face of the moon will be always nearly turned to
the upper focus of its orb; but, as the situation of that focus requires, will
deviate a little to one side and to the other from the earth in the lower
focus; and this is the libration in longitude ; for the libration in latitude
arises from the moon’s latitude, and the inclination of its axis to the plane
of the ecliptic. This theory of the libration of the moon, Mr. N. Mercator

OF NATURAL PHILOSOPHY.

405

Book III.]

in his Astronomy, published at the beginning of the year 1676, explained
more fully out of the letters I sent him. The utmost satellite of Saturn
seems to revolve about its axis with a motion like this of the moon, respect¬
ing Saturn continually with the same face; for in its revolution round
Saturn, as often as it comes to the eastern part of its orbit, it is scarcel}
visible, and generally quite disappears; which is like to be occasioned by
some spots in that part of its body, which is then turned towards the earth,
asM. Cassini has observed. So also the utmost satellite of Jupiter seems
to revolve about its axis with a like motion, because in that part of its body
which is turned from Jupiter it has a spot, which always appears as if it
were in Jupiter’s own body, whenever the satellite passes between Jupiter
and our eye.

PROPOSITION XVIII. THEOREM XVI.

*

That the axes of the planets are less than the diameters drawn perpen¬
dicular to the axes.

The equal gravitation of the parts on all sides would give a spherical
figure to the planets, if it was not for their diurnal revolution in a circle.
By that circular motion it comes to pass that the parts receding from the
axis endeavour to ascend about the equator; and therefore if the matter is
in a fluid state, by its ascent towards the equator it will enlarge the di¬
ameters there, and by its descent'towards the poles it will shorten the axis.
So the diameter of Jupiter (by the concurring observations of astronomers)
is found shorter betwixt pole and pole than from east to west. And, by
the same argument, if our earth was not higher about the equator than at
the poles, the seas would subside about the poles, and, rising towards the
equator, would lay all things there under water.

PROPOSITION XIX. PROBLEM III

To find the proportion of the axis of a planet to the diameter j perpen¬
dicular thereto.

Our countryman, Mr. Norwood, measuring a distance of 905751 feet of
London measure between London and York, in 1635, and observing the
difference of latitudes to be 2° 28', determined the measure of one degree
to be 367196 feet of London measure, that is 57300 Paris toises. M
Picart, measuring an arc of one degree, and 22' 55" of the meridian be¬
tween Amiens and Malvoisine , found an arc of one degree to be 57060
Paris toises. M. Cassini, the father, measured the distance upon the me¬
ridian from the town of Collioure in Roussillon to the Observatory of
Pari»; and his son added the distance from the Observatory to the Cita¬
del of Dunkirk. The whole distance was 486156£ toises and the differ
encc of the latitudes of Collioure and Dunkirk was 8 degrees, and 31

106

THE MATHEMATICAL PRINCIPLES

[Book 111,

Ilf". Hence an arc of one degree appears to be 57061 Paris toises.
And from these measures we conclude that the circumference of the earth
is 123249600, and its semi-diameter 19615800 Paris feet, upon the sup¬
position that the earth is of a spherical figure.

In the latitude of Paris a heavy body falling in a second of time de¬
scribes 15 Paris feet, l inch, If line, as above, that is, 2173 lines f. The
weight of the body is diminished by the weight of the ambient air. Let
us suppose the weight lost thereby to be TT foo P art °f the whole weight;
then that heavy body falling in vacuo will describe a height of 2174 lines
in one second of time.

A body in every sidereal day of 23'*. 56' 4" uniformly revolving in a
circle at the distance of 19615S00 feet from the centre, in one second of
time describes an arc of 1433,46 feet; the versed sine of which is 0,05236561
feet, or 7,54064 lines. And therefore the force with which bodies descend
in the latitude of Paris is to the centrifugal force of bodies in the equator
arising from the diurnal motion of the earth as 2174 to 7,54064.

The centrifugal force of bodies in the equator is to the centrifugal force
with which bodies recede directly from the earth in the latitude of Paris
4S° 50' 10" in the duplicate proportion of the radius to the cosine of the
latitude, that is, as 7,54064 to 3,267. Add this force to the force with
which bodies descend by their weight in the latitude of Paris , and a body,
in the latitude of Paris , falling by its whole undiminished force of gravity,
in the time of one second, will describe 2177,267 lines, or 15 Paris feet,
1 inch, and 5,267 lines. And the total force of gravity in that latitude
will be to the centrifugal force of bodies in the equator of the earth as
2177,267 to 7,54064, or as 289 to 1.

r

\ c

Wherefore if APBQ represent the figure of the
earth, now no longer spherical, but generated by the
rotation of an ellipsis about its lesser axis PQ: and
ACQqca a canal full of water, reaching from the pole
Qq to the centre Cc, and thence rising to the equator
A a ; the weight of the water in the leg of the canal
AGca will be to the weight of water in the other leg

QiGcq as 289 to 288, because the centrifugal force arising from the circu¬
lar motion sustains and takes off one of the 2S9 parts of the weight (in the

one leg), and the weight of 2S8 in the other sustains the rest. But by
computation (from Cor. 2, Prop. XCI, Book I) I find, that, if the matter
—-of the earth was all uniform, and without any motion, and its axis PQ,

were to the diameter AB as 100 to 101, the force of gravity in the

place Q towards the earth would be to the force of gravity in the same
place Q towards a sphere described about the centre C with the radius
PC, or QC, as 126 to 125. And, by the same argument, the force of
gravity in the place A towards the spheroid generated by the rotation of

OF NATURAL PHILOSOPHY.

407

Book III.]

the ellipsis APBQ about the axis AB is to the force of gravity in the
same place A, towards the sphere described about the centre C with the
radius AO, as 125 to 126. But the force of gravity in the place A to¬
wards the earth is a mean proportional betwixt the forces of gravity to¬
wards the spheroid and this sphere; because the sphere, by having its di¬
ameter PQ diminished in the proportion of 101 to 100, is transformed into
the figure of the earth; and this figure, by having a third diameter per¬
pendicular to the two diameters AB and PQ, diminished in the same pro¬
portion, is converted into the said spheroid ; and the force of gravity in A,
in either case, is diminished nearly in the same proportion. Therefore the
force of gravity in A towards the sphere described about the centre C with
the radius AC, is to the force of gravity in A towards the earth as 126 to
1251. And the force of gravity in the place Q towards the sphere de¬
scribed about the centre C with the radius QC, is to the force of gravity
in the place A towards the sphere described about the centre C, with the
radius AC, in the proportion of the diameters (by Prop. LXXII, Book I),
that is, as 100 to 101. If, therefore, we compound those three proportions
126 to 125, 126 to 125}. and 100 to 101, into one, the force of gravity in
the place Q towards the earth will be to the force of gravity in the place
A towards the earth as 126 X 126 X 100 to 125 X 125} X 101; or as
501 to 500.

Now since (by Cor. 3, Prop. XCI, Book I) the force of gravity in either
leg of the canal AC ca, or QC cq, is as the distance of the places from the
centre of the earth, if those legs are conceived to be divided by transverse,
parallel, and equidistant surfaces, into parts proportional to the wholes,
the weights of any number of parts in the one leg AC ca will be to the
weights of the same number of parts in the other leg as their magnitudes
and the accelerative forces of their gravity conjunctly, that is, as 101 to
100, and 500 to 501, or as 505 to 501. And therefore if the centrifugal
force of every part in the leg AC ca, arising from the diurnal motion, was
to the weight of the same part as 4 to 505, so that from the weight of
every part, conceived to be divided into 505 parts, the centrifugal force
might take off four of those parts, the weights would remain equal in each
leg, and therefore the fluid would rest in an equilibrium. But the centri¬
fugal force of every part is to the weight of the same part as 1 to 289 ;
that is, the centrifugal force, which should be parts of the weight, is
only j}g part thereof. And, therefore, I say, by the rule of proportion,
that if the centrifugal force make the height of the water in the leg
AC ca to exceed the height of the water in the leg QC cq by one part
of its whole height, the centrifugal force will make the excess of the
height in the leg AC ca only part of the height of the water in the
other leg QC cq ; and therefore the diameter of the earth at the equator, is
to its diameter from pole to pole as 230 to 229. And since the mean semi-

THE MATHEMATICAL PRINCIPLES

10S

|Book III.

diameter of the earth, according to Picarfs mensuration, is 19615S00
Paris feet, or 3923,16 miles (reckoning 5000 feet to a mile), the earth
will be higher at the equator than at the poles by 85472 feet, or 17^-
miles. And its height at the equator will be about 1965S600 feet, and at
the poles 19573000 feet.

If, the density and periodic time of the diurnal revolution remaining the
same, the planet was greater or less than the earth, the proportion of the
centrifugal force to that of gravity, and therefore also of the diameter be¬
twixt the poles to the diameter at the equator, would likewise remain the
3 ame. But if the diurnal motion was accelerated or retarded in any pro¬
portion, the centrifugal force would be augmented or diminished nearly in
the same duplicate proportion ; and therefore the difference of the diame¬
ters will be increased or diminished in the same duplicate ratio very nearly.
And if the density of the planet was augmented or diminished in any pro¬
portion, the force of gravity tending towards it would also be augmented
or diminished in the same proportion: and the difference of the diameters
contrariwise would be diminished in proportion as the force of gravity is
augmented, and augmented in proportion as the force of gravity is dimin¬
ished. Wherefore, since the earth, in respect of the fixed stars, revolves in
23 h . 56', but Jupiter in 9 h . 56', and the squares of their periodic times are
as 29 to 5, and their densities as 400 to 94 the difference of the diameters

29 400 1

of Jupiter will be to its lesser diameter as -jr X X ^9 to ^ or as 1

9 j, nearly. Therefore the diameter of Jupiter from east to west is to its
diameter from pole to pole nearly as 10 J- to 9^. Therefore since its
greatest diameter is 37", its lesser diameter lying between the poles will
be 33" 25'". Add thereto about 3" for the irregular refraction of light,
and the apparent diameters of this planet will become 40" and 36" 25"';
which are to each other as 11 1 to 10£, very nearly. These things are so
upon the supposition that the body of Jupiter is uniformly dense. But
now if its body be denser towards the plane of the equator than towards
the poles, its diameters may be to each other as 12 to 11, or 13 to 12 or
perhaps as 14 to 13.

And Cassini observed in the year 1691, that the diameter of Jupiter
reaching from east to west is greater by about a fifteenth part than the
other diameter. Mr. Pound with his 123 feet telescope, and an excellent
micrometer, measured the diameters of Jupiter in the year 1719, and found |
them as follow.

The Times.

Greatest diam.

Lesser diam.

The diam. to each other.

Day. Hours.

January 28 6

March 6 7

March 9 7

April 9 9

Parts

13,40

13,12

13,12

1*2,32

Parts

12,28

12,20

12,08

11,48

As 12 to 11
13| to 12!
12f to Ilf
14d to 13d|

OF NATURAL PHILOSOPHY.

409

(4ook 111.]

So that the theory agrees with the phenomena; for the planets are more
heated by the sun’s rays towards their equators, and therefore are a lit £e
more condensed by that heat than towards their poles.

Moreover, that there is a diminution of gravity occasioned by the diur¬
nal rotation of the earth, and therefore the earth rises higher there than it
does at the poles (supposing that its matter is uniformly dense), will ap¬
pear by the experiments of pendulums related under the following Propo¬
sition.

£±CC

PROPOSITION XX. PROBLEM IV.

To find and compare together the weights of bodies in the different re¬
gions of our earth.

Because the weights of the unequal legs of the canal
of water ACQ, qca are equal; and the weights of the
parts proportional to the whole legs, and alike situated
in them, are one to another as the weights of the Pj
wholes, and therefore equal betwixt themselves; the
weights of equal parts, and alike situated in the legs,
will be reciprocally as the legs, that is, reciprocally as
230 to 229. And the case is the same in all homogeneous equal bodies alike
situated in the legs of the canal. Their weights are reciprocally as the legs,
that is, reciprocally as the distances of the bodies from the centre of the earth.
Therefore if the bodies are situated in the uppermost parts of the canals, or on
the surface of the earth, their weights will be one to another reciprocally as
their distances from the centre. And, by the same argument, the weights in
all other places round the whole surface of the earth are reciprocally as the
distances of the places from the centre; and, therefore, in the hypothesis
of the earth’s being a spheroid are given in proportion.

Whence arises this Theorem, that the increase of weight in passing from
tne equator to the poles is nearly as the versed sine of double the latitude;
or, which comes to the same thing, as the square of the right sine of the
latitude; and the arcs of the degrees of latitude in the meridian increase
nearly in the same proportion. And, therefore, since the latitude of Paris
is 48° 50', that of places under the equator 00° 00', and that of places
under the poles 90°; and the versed sines of double those arcs are
11334,00000 and 20000, the radius being 10000; and the force of gravity
at the pole is to the force of gravity at the equator as 230 to 229; and
the excess of the force of gravity at the pole to the force of gravity at the
equator as 1 to 229 ; the excess of the force of gravity in the latitude of
Paris will be to the force of gravity at the equator asl X Hfirt t- 0 ^29,
or as 5667 to 2290000. And therefore the whole forces of gravity in
those places will be one to the other as 2295667 to 2290000. Wherefore
since the lengths of pendulums vibrating in equal times are as the forces of

410

THE MATHEMATICAL PRINCIPLES

[Book III.

gravity, and in the latitude of Paris , the length of a pendulum vibrating
seconds is 3 Paris feet, and lines, or rather because of the weight ol
the air, 8f lines, the length of a pendulum vibrating in the same time
ander the equator will be shorter by 1,087 lines. And by a like calculus
the following table is made.

Latitude of
the place.

Length of the
pendulum

Measure of one degree
in the meridian.

Deg.

Feet

Lines.

Toises.

0

3 .

7,468

56637

5

3 .

7,482

56642

10

3 .

7,526

56659

15

3 .

7,596

56687

20

3 .

7,692

56724

25

3 .

7,812

56769

30

3 .

7,948

56823

35

3 .

8,099

56882

40

3 .

8,261

56945

1

3 .

8,294

5695?

2

3 .

8,327

56971 [

3

3 .

8,361

56984

4

3 .

8,394

56997

45

3 .

8,428

57010

6

3 .

8,461

57022

7

3 .

8,494

57035

8

3 .

8,528

57048

9

1 3 .

8,561

57061

50

3 .

8,594

57074

55

3 .

8,756

57137

60

3 .

8^907

57196

65

3 .

9,044

57250

70

3 .

9,162

57295

75

3 .

9,258

57332

80

3 .

9,329

57360

85

3 .

9,372

57377

90

3 .

9,387

57382

By this table, therefore, it appears that the inequality of degrees is sc
small, that the figure of the earth, in geographical matters, may be con¬
sidered as spherical; especially if the earth be a little denser towards the
plane of the equator than towards the poles.

Now several astronomers, sent into remote countries to make astronomical
observations, have found that pendulum clocks do accordingly move slower
near the equator than in our climates. And, first of all, in the year l 72,
M. Richer took notice of it in the island of Cayenne ; for when, in the
month of A ugust, he was observing the transits of the fixed stars over the
meridian, he found his clock to go slower than it ought in respect of the
mean motion of the sun at the rate of 2' 28" a day. Therefore, fitting up
a simple pendulum to vibrate in seconds, which were measured by an ex¬
cellent clock, he observed the length of that simple pendulum ; and this he
did over and over every week for ten months together. And upon his re¬
turn to France, comparing the length of that pendulum with the length

Book ilJ.j

OF NATURAL PHILOSOPHY.

411

of the pendulum at Paris (which was 3 Paris feet and S§ lines), he found
it shorter by lj line.

Afterwards, our friend Dr. Halley, about the year 1677, arriving at the
island of St. Helena, found his pendulum clock to go slower there than at
ljondou without marking the difference. But he shortened the rod of
his clock by more than the j of an inch, or It line; and to effect this, be¬
cause the length of the screw at the lower end of the rod was not sufficient,
he interposed a wooden ring betwixt the nut and the ball.

Then, in the year 1682, M. Varin and M. des Hayes found the length
of a simple pendulum vibrating in seconds at the Royal Observatory of
Paris to be 3 feet and Sf lines. And by the same method in the island
of Goree, they found the length of an isochronal pendulum to be 3 feet and
61 lines, differing from the former by two lines. And in the same year,
going to the islands of Guadaloupe and Martinico, they found that the
length of an isochronal pendulum in those islands was 3 feet and 6^ lines.

After this, M. Couplet, the son, in the month of July 1697, at the Royal
Observatory of Paris , so fitted his pendulum clock to the mean motion of
the sun, that for a considerable time together the clock agreed with the
motion of the sun. In November following, upon his arrival at Lisbon, he
found his clock to go slower than before at the rate of 2' 13" in 24 hours.
And next March coming to Paraiba, he found his clock to go slower than
at Paris, and at the rate 4' 12" in 24 hours; and he affirms, that the pen¬
dulum vibrating in seconds was shorter at Lisbon by '2\ lines, and at Pa¬
raiba by 3| lines, than at Paris. He had done better to have reckoned
those differences l£ and 2f: for these differences correspond to the differ¬
ences of the times 2' 13" and 4' 12". But this gentleman’s observations
are so gross, that we cannot confide in them.

In the following years, 1699, and 1700, M. des Hayes , making another
voyage to America, determined that in the island of Cayenne and Granada
the length of the pendulum vibrating in seconds was a small matter less
than 3 feet and 6| lines; that in the island of St. Christophers it was
3 feet and 6f lines; and in the island of St. Domingo 3 feet and 7
lines.

And in the year 1704, P. Feuille , at Puerto Bello in America, found
that the length of the pendulum vibrating in seconds was 3 Paris feet,
and only 5 T 7 ¥ lines, that is, almost 3 lines shorter than at Paris ; but the
observation was faulty. For afterward, going to the island of Martinico,
he found the length of the isochronal pendulum there 3 Paris feet and
5\ | lines.

Now the latitude of Paraiba is 6° 38' south; that of Puerto Bello 9°
33' north; and the latitudes of the islands Cayenne, Goree, Gaudaloupe ,
Martinico, Granada, St. Christophers, and St. Domingo, are respectively
4 C 55', 14° 40", 15° 00', 14° 44', 12° 06', 17° 19',and 19° 48',north. AnJ

412

THE MATHEMATICAL PRINCIPLES

rBooK in

the excesses of the length of the pendulum at Paris above the lengths of
the isochronal pendulums observed in those latitudes are a little greater
than by the table of the lengths of the pendulujn before computed. And
therefore the earth is a little higher ,under the equator than by the prece¬
ding calculus, and a little denser at the centre than in mines near the sur¬
face, unless, perhaps, the heats of the torrid zone have a little extended the
length of the pendulums.

For M. Picart has observed, that a rod of iron, which in frosty weather
in the winter season was one foot long, when heated by lire, was lengthened
into one foot and line. Afterward M. de la Hire found that a rod of
iron, which in the like winter season was 6 feet long, when exposed to the
heat of the summer sun, was extended into 6 feet and § line. In the former
case the heat was greater than in the latter; but in the latter it was greater
than the heat of the external parts of a human body; for metals exposed
to the summer sun acquire a very considerable degree of heat. But the rod
of a pendulum clock is never exposed to the heat of the summer sun, nor
ever acquires a heat equal to that of the external parts of a human body ;
and, therefore, though the 3 feet rod of a pendulum clock will indeed be a
little longer in the summer than in the winter season, yet the difference will
scarcely amount to i line. Therefore the total difference of the lengths of
isochronal pendulums in different climates cannot be ascribed to the differ¬
ence of heat; nor indeed to the mistakes of the French astronomers. For
although there is not a perfect agreement betwixt their observations, yet
the errors are so small that they may be neglected; and in this they all
agree, that isochronal pendulums are shorter under the equator than
at the Royal Observatory of Paris , by a difference not less than If line,
nor greater than 2f lines. By the observations of M. Richer , in the island
of Cayenne, the difference was If line. That difference being corrected by
those of M. des Hayes, becomes If line or If line. By the less accurate
observations of others, the same was made about two lines. And this dis¬
agreement might arise partly from the errors of the observatiuns, partly
from the dissimilitude of the internal parts of the earth, and the height of
mountains; partly from the different heats of the air.

I take an iron rod of 3 feet long to be shorter by a sixth part of one line
in winter time with us here in England than in the summer. Because of
the great heats under the equator, subduct this quantity from the difference
of one line and a quarter observed by M. Richer, and there will remain one
line yV, which agrees very well with l T -ofo line collected, by the theory a
little before. M. Richer repeated his observations, made in the island of
Cayenne, every week for ten months together, and compared the lengths of
the pendulum which he had there noted in the iron rods with the lengths
thereof which he observed in France. This diligence and care seems to
have been wanting to the other observers. If this gentleman’s observations

OF NATURAL PHILOSOPHY.

413

Book III.J

are to be depended on, the earth is higher under the equator than at the
poles, and that by an excess of about 17 miles; as appeared above by the
theory.

PROPOSITION XXI. THEOREM XVII.

That the equinoctial points go backward , and that the axis of the earth ,
by a, nutation in every annual revolution , twice vibrates towards the
ecliptic , and as ojteu returns to its former position.

The proposition appears from Cor. 20, Prop. LXVI, Book I; but
that motion of nutation must be very small, and, indeed, scarcely per¬
ceptible.

PROPOSITION XXII. THEOREM XVIII.

That all the motions of the moon , and all the inequalities of those motions ,
follow from the principles which we have laid down.

That the greater planets, while they are carried about the sun, may in
the mean time carry other lesser planets, revolving about them; and that
those lesser planets must move in ellipses which have their foci in the cen¬
tres of the greater, appears from Prop. LXV, Book I. But then their mo¬
tions will be several ways disturbed by the action of the sun, and they will
sulfer such inequalities as are observed in our moon. Thus our moon (by
Cor. 2, 3, 4, and 5, Prop. LXVI, Book I) moves faster, and, by a radius
drawn to the earth, describes an area greater for the time, and has its orbit
less curved, and therefore approaches nearer to the earth in the syzygies
than in the quadratures, excepting in so far as these eifects are hindered by
the motion of eccentricity; for (by Cor. 9, Prop. LXVI, Book I) the eccen¬
tricity is greatest when the apogeon of the moon is in the syzygies, and
least when the same is in the quadratures; and upon this account the pe-
rigeon moon is swifter, and nearer to us, but the apogeon moon slower,
and farther from us, in the syzygies than in the quadratures. Moreover,
the apogee goes forward, and the nodes backward; and this is done not with
a regular but an unequal motion. For (by Cor. 7 and S, Prop. LXVI,
Book I) the apogee goes more swiftly forward in its syzygies, more slowly
backward in its quadratures; and, by the excess of its progress above its
regress, advances yearly in consequents. But, contrariwise, the nodes (by
Cor. 11, Prop. LXVI, Book I) are quiescent in their syzygies, and go fastest
back in their quadratures. Farther, the greatest latitude of the moon (by
Cor. 10, Prop. LXVI, Book I) is greater in the quadratures of the moon
than in its syzygies. And (by Cor. 6, Prop. LXVI, Book I) the mean mo¬
tion of the moon is slower in the perihelion of the earth than in its aphelion.
And these are the principal inequalities (of the moon) taken notice of by
astronomers.

414

THE MATHEMATICAL PRINCIPLES

[Book Ill

But there are yet other inequalities not observed by former astronomers,
by which the motions of the moon are so disturbed, that to this day we
have not been able to bring them under any certain rule. For the veloc¬
ities or horary motions of the apogee and nodes of the moon, and their
equations, as well as the difference betwixt the greatest eccentricity in the
syzygics, and the least eccentricity in the quadratures, and that inequality
which we call the variation, are (by Cor. 14, Prop. LXYI, Book I) in the
course of the year augmented and diminished in the triplicate proportion
of the sun’s apparent diameter. And besides (by Cor. 1 and 2, Lem. 10,
and Cor. 16, Prop. LXYI, Book I) the variation is augmented and
diminished nearly in the duplicate proportion of the time between
the quadratures. But in astronomical calculations, this inequality
is commonly thrown into and confounded with the equation of the moon’s
centre.

PROPOSITION XXIII. PROBLEM Y.

To derive the unequal motions of the satellites of Jupiter and Saturn
from the motions of our moon.

From the motions of our moon we deduce the corresponding motions of
the moons or satellites of Jupiter in this manner, by Cor. 16, Prop. LXYI,
Book l. The mean motion of the nodes of the outmost satellite of Jupiter
is to the mean motion of the nodes of our moon in a proportion compound¬
ed of the duplicate proportion of the periodic times of the earth about the
sun to the periodic times of Jupiter about the sun, and the simple propor¬
tion of the periodic time of the satellite about Jupiter to the periodic time
of our moon about the earth; and, therefore, those nodes, in the space of
a hundred years, are carried 8° 24' backward, or in antecedentia. The
mean motions of the nodes of the inner satellites are to the mean motion of
the nodes of the outmost as their periodic times to the periodic time of the
former, by the same Corollary, and are thence given. And the motion of
the apsis of every satellite in consequents is to the motion of its nodes in
antecedentia as the motion of the apogee of our moon to the motion of its
nodes (by the same Corollary), and is thence given. But the motions of
the apsides thus founds must be diminished in the proportion of 5 to 9, or
of about 1 to 2, on account of a cause which I cannot here descend to ex¬
plain. The greatest equations of the nodes, and of the apsis of every satel¬
lite, are to the greatest equations of the nodes, and apogee of our moon re¬
spectively, as the motions of the nodes and apsides of the satellites, in the
time of one revolution of the former equations, to the motions of the nodes
and apogee of our moon, in the time of one revolution of the latter equa¬
tions. The variation of a satellite seen from Jupiter is to the variation of
mir moon in the same proportion as the whole motions of their nodes

Book I1IJ

OF NATURAL PHILOSOPHY.

415

respectively during the times in which the satellite and our moon (after
parting from) are revolved (again) to the sun, by the same Corollary; and
therefore in the outmost satellite the variation does not exceed 5" 12'".

PROPOSITION XXIV. THEOREM XIX.

That the flax and rejiax of the sea arise from the actions oj the sun

and moon.

By Cor. 19 and 20, Prop. LXVI, Book I, it appears that the waters of
the sea ought twice to rise and twice to fall every day, as well lunar as solar;
and that the greatest height of the waters in the open and deep seas ought
to follow the appulse of the luminaries to the meridian of the place by a
less interval than 6 hours ; as happens in all that eastern tract of the Atlantic
and JEthiopic seas between France and the Cape of Good Hope ; and on
the coasts of Chili and Peru in the South Sea ; in all which shores the
Ho »d falls out about the second, third, or fourth hour, unless where the
motion ^propagated from the deep ocean is by the shallowness of the cham
nels, through which it passes to some particular places, retarded to the
fifth, sixth, or seventh hour, and even later. The hours I reckon from the
appulse of each luminary to the meridian of the place, as well under as
above the horizon ; and by the hours of the lunar day I understand the
24th parts of that time which the moon, by its apparent diurnal motion,
employs to come about again to the meridian of the place which it left the
day before. The force of the sun or moon in raising the sea is greatest in
the appulse of the luminary to the meridian of the place; but the force
impressed upon the sea at that time continues a little while after the im¬
pression, and is afterwards increased by a new though less force still act¬
ing upon it. This makes the sea rise higher and higher, till this new force
becoming too weak to raise it any more, the sea rises to its greatest height.
And this will come to pass, perhaps, in one or two hours, but more fre¬
quently near the shores in about three hours, or even more, where the sea
is shallow.

The two luminaries excite two motions, which will not appear distinctly,
but between them will arise one mixed motion compounded out of both.
In the conjunction or opposition of the luminaries their forces will be con¬
joined, and bring on the greatest flood and ebb. In the quadratures the
sun will raise the waters which the moon depresses, and depress the waters
which the moon raises, and from the difference of their forces the smallest
of all tides will follow r . And because (as experience tells us) the force of
the moon is greater than that of the sun, the greatest height of the waters
will happen about the third lunar hour. Out of the syzygies and quadra¬
tures, the greatest tide, which by the single force of the moon ought to fall
out at the third lunar hour, and by the single force of the sun at the third
solar hour, by the compounded forces of both must fall out in an interme*

416

THE MATHEMATICAL PRINCIPLES

[Book JIJ

diate time that aproaches nearer to the third hour of the moon than tc
that of the sun. And, therefore, while the moon is passing from the syzy*
gies to the quadratures, during which time the 3d hour of the sun precedes
the 3d hour of the moon, the greatest height of the waters will also precede
the 3d hour of the moon, and that, by the greatest interval, a little after
the octants of the moon; and, by like intervals, the greatest tide will fol¬
low the 3d lunar hour, while the moon is passing from the quadratures to
the syzygies. Thus it happens in the open sea; for in the mouths of
rivers the greater tides come l iter to their height.

But the effects of the luminaries depend upon their distances from the
earth ; for when they are less distant, their effects are greater, and when
more distant, their effects are less, and that in the triplicate proportion of
their apparent diameter. Therefore it is that the sun, in the winter time,
being then in its perigee, has a greater effect, and makes the tides in the
syzygies something greater, and those in the quadratures something less
than in the summer season; and every month the moon, while in the peri¬
gee, raises greater tides than at the distance of 15 days before or after,
when it is in its apogee. Whence it comes to pass that two highest
tides do not follow one the other in two immediately succeeding syzygies.

The effect of either luminary doth likewise depend upon its declination
or distance from the equator ; for if the luminary was placed at the pole,
it would constantly attract all the parts of the waters without any inten¬
sion or remission of its action, and could cause no reciprocation of motion.
And, therefore, as the luminaries decline from the equator towards either
pole, they will, by degrees, lose their force, and on this account will excite
lesser tides in the solstitial than in the equinoctial syzygies. But in the
solstitial quadratures they will raise greater tides than in the quadratures
about the equinoxes; because the force of the moon, then situated in the
equator, most exceeds the force of the sun. Therefore the greatest tides
fall out in those syzygies, and the least in those quadratures, which hap¬
pen about the time of both equinoxes : and the greatest tide in the syzy¬
gies is always succeeded by the least tide in the quadratures, as we find
by experience. But, because the sun is less distant from the earth in
winter than in summer, it comes to pass that the greatest and least tides

more frequently appear before than after the vernal equinox, and more
frequently after than before the autumnal.

Moreover, the effects of the lumi¬
naries depend upon the latitudes of
places. Let AjoEP represent the
earth covered with deep waters; C
its centre; P ,p it3 poles; AE the
equator; F any place without the
equator; F f the parallel of the place;
T)d the correspondent parallel on the

OF NATURAL PHILOSOPHY.

417

Book III.]

other side of the equator; L the place of the moon three Lours before;
H the place of the earth directly under it; h the opposite place ; K, k the
places at 90 degrees distance; CH, C h, the greatest heights of the sea
from the centre of the earth ; and CK, C£, its least heights: and if with
the axes H//, Kk, an ellipsis is described, and by the revolution of that
ellipsis about its longer axis Wi a spheroid HPK hpk is formed, this sphe¬
roid will nearly represent the figure of the sea; and CP, Of, CD, C d,
will represent the heights of the sea in the places F/, D d. But far¬
ther ; in the said revolution of the ellipsis any point N describes the circle
NM cutting the parallels F f, D d, in any places RT, and the equator AE
in S ; CN will represent the height of the sea in all those places R, S,
T, situated in this circle. Wherefore, in the diurnal revolution of any
place F, the greatest flood will be in F, at the third hour after the appulse
of the moon to the meridian above the horizon; and afterwards the great¬
est ebb in Q,, at the third hour after the setting of the moon ; and then
the greatest flood in f, at the third hour after the appulse of the moon to
the meridian under the horizon ; and, lastly, the greatest ebb in Q,, at the
third hour after the rising of the moon ; and the latter flood in f will be
less than the preceding flood in F. For the whole sea is divided into two
hemispherical floods, one in the hemisphere KH/j on the north side, the
other in the opposite hemisphere lUik, which we may therefore call the
northern and the southern floods. These floods, being always opposite the one
to the other, come by turns to the meridians of all places, after an interval
of 12 lunar hours. And seeing the northern countries partake more of
the northern flood, and the southern countries more of the southern flood,
thence arise tides, alternately greater and less in all places without the
equator, in which the luminaries rise and set. But the greatest tide will
happen when the moon declines towards the vertex of the place, about the
third hour after the appulse of the moon to the meridian above the hori¬
zon ; and when the moon changes its declination to the other side of the
equator, that which was the greater tide will be changed into a lesser.
And the greatest difference of the floods will fall out about the times of
the solstices; especially if the ascending node of the moon is about the
first of Aries. So it is found by experience that the morning tides in
winter exceed those of the evening, and the evening tides in summer ex¬
ceed those of the morning; at Plymouth by the height of one foot, but at
Bristol by the height of 15 inches, according to the observations of Cole-
press and Sturmy.'

But the motions which we have been describing suffer some alteration
from that force of reciprocation, which the waters, being once moved, retain
a little while by their vis insita. Whence it comes to pass that the tides
may continue for some time, though the actions of the luminaries should

27

418

THE MATHEMATICAL PRINCIPLES

[Book III

oease. This power of retaining the impressed motion lessens the difference
■>f the alternate tides, and makes those tides which immediately succeed
after the syzygies greater, and those which follow next after the quadra¬
tures less. And hence it is that the alternate tides at Plymouth and
Bristol do not differ much more one from the other than by the height of
a foot or 15 inches, and that the greatest tides of all at those ports are not
the first but the third after the syzygies. And, besides, all the motions are
retarded in their passage through shallow channels, so that the greatest
tides of all, in some straits and mouths of rivers, are the fourth or even the
fifth after the syzygies.

Farther, it may happen that the tide may be propagated from the ocean
through different channels towards the same port, and may pass quicker
through some channels than through others; in which case the same tide,
divided into two or more succeeding one another, may compound new mo¬
tions of different kinds. 1 iet us suppose two equal tides flowing towards
the same port from different places, the one preceding the other by 6 hours;
and suppose the first tide to happen at the third hour of the appulse of the
moon to the meridian of the port. If the moon at the time of the appulse
to the meridian was in the equator, every 6 hours alternately there would
arise equal floods, which, meeting with as many equal ebbs, would so bal¬
ance one the other, that for that day, the water would stagnate and remain
quiet. If the moon then declined from the equator, the tides in the ocean
would be alternately greater and less, as was said; and from thence two
greater and two lesser tides would be alternately propagated towards that
port. But the two greater floods would make the greatest height of the
waters to fall out in the middle time betwixt both; and the greater and
lesser floods would make the waters to rise to a mean height in the middle
time between them, and in the middle time between the two lesser floods the
waters would rise to their least height. Thus in the space of 24 hours the
waters would come, not twice, as commonly, but once only to their great¬
est, and once only to their least height; and their greatest height, if the
moon declined towards the elevated pole, would happen at. the 6th or 30th
hour after the appulse of the moon to the meridian; and when the moon
changed its declination, this flood would be changed into an ebb. An ex¬
ample of all which Dr. Halley has given us, from the observations of sea¬
men in the port of Batsham, in the kingdom of Timquin , in the latitude
of 20° 50' north. In that port, on the day which follows after the passage
of the moon over the equator, the waters stagnate: when the moon declines
to the north, they begin to flow and ebb, not twice, as in other ports, but
once only everyday : and the flood happens at the setting, and the greatest
ebb at the rising of the moon. This tide increases with the declination of
the moon till the 7 th or 8th day; then for the 7 or 8 days following it

Book III.]

of natural philosophy.

419

decreases at the same rate as it had increased before, and ceases when the
moon changes its declination, crossing over the equator to the south. Af¬
ter which the flood is immediately changed into an ebb; and thenceforth
the ebb happens at the setting and the flood at the rising of the moon; till
the moon, again passing the equator, changes its declination. There are
two inlets to this port and the neighboring channels, one from the seas of
China , between the continent and the island of Lenconia ; the other from
the Indian sea, between the continent and the island of Borneo. But
whether there be really two tides propagated through the said channels, one
from the Indian, sea in the space of 12 hours, and one from the sea of
China in the space of 6 hours, which therefore happening at the 3d and
9th lunar hours, by being compounded together, produce those motions; or
whether there be any other circumstances in the state,of those seas, I leave
to be determined by observations on the neighbouring shores.

Thus I have explained the causes of the motions of the moon and of the
sea. Now it is fit to subjoin something concerning the quantity of those
motions.

PROPOSITION XXV. PROBLEM VI.

To find the forces with ichich the sun disturbs the motions of the moon.

Let S represent the sun, T the
earth, P the moon, CADB the
moon’s orbit. In SP take SK
equal to ST; and let SL be to
SK in the duplicate proportion
of SK to SP: draw LM parallel
to PT; and if ST or SK is sup¬
posed to represent the accelerated force of gravity of the earth towards the
sun, SL will represent the accelerative force of gravity of the moon towards
the sun. But that force is compounded of the parts SM and LM, of which
the force LM, and that part of SM which is represented by TM, disturb
the motion of the moon, as we have shewn in Prop. LXYI, Book I, and
its Corollaries. Forasmuch as the earth and moon are revolved about
their common centre of gravity, the motion of the earth about that centre
will be also disturbed by the like forces; but we may consider the sums-
both of the forces and of the motions as in the moon, and represent the sum-
of the forces by the lines TM and ML, which are analogous to them both.
The force ML (in its mean quantity) is to the centripetal force by which
the moon may be retained in its orbit revolving about the earth at rest, at
the distance P I, in the duplicate proportion of the periodic time of the
moon about the earth to the periodic time of the earth about the sun (by
Cor. 17, Prop. LXVI, Book I); that is, in the duplicate proportion of 27 «*.
7 \ 43' to 365 1 .6“. 9'; or as 1000 to 178725; or as 1 to 178Jf. But in the

120

THE MATHEMATICAL PRINCIPLES

[Book Ill

4 th Prop, of this Book we found, that, if both earth and moon were revolved
rfoout their common centre of gravity, the mean distance of the one from
the other would be nearly 60| mean semi-diameters of the earth; and the
force by which the moon may be kept revolving in its orbit about the earth
in rest at the distance PT of 60^ semi-diameters of the earth, is to the
force by which it may be revolved in the same time, at the distance of 60
semi-diameters, as 60^- to 60: and this force is to the force of gravity with
us very nearly as l to 60 X 60. Therefore the mean force ML is to the
force of gravity on the surface of our earth as 1 X 60-*- to 60 X 60 X 60
X 178|f, or as 1 to 63S092,6 ; whence by the proportion of the lines TM,
ML, the force TM is also given; and these are the forces with which the
sun disturbs the motions of the moon. Q.E.I.

PROPOSITION XXVI. PROBLEM VII.

To find the horary increment of the area which the moon , by a radius
drawn to the earth , describes in a circular orbit.

W--. im-—^ We have above

shown that the area
which the moon de¬
scribes by a radius
drawn to the earth
is proportional to

.

K \

"'x'' \

X

L_\

T

\ /

IB M

\ \x' / tion, excepting in so

\ % \. J far as the moon’s

\ / motion is disturbed

/ by the action of the

sun; and here we
D propose to investi¬

gate the inequality of the moment, or horary increment of that area or
motion, so disturbed. To render the calculus more easy, we shall suppose
the orbit of the moon to be circular, and neglect all inequalities but that

only which is now under consideration; and, because of the immense dis¬

tance of the sun, we shall farther suppose that the lines SP and ST are
parallel. By this means, the force LM will be always reduced to its mean
quantity TP, as well as the force TM to its mean quantity 3PK. These
forces (by Cor. 2 of the Laws of Motion) compose th£ force TL; and
this force, by letting fall the perpendicular LE upon the radius TP, is
resolved into the forces TE,EL; of which the forceTE,acting constantly
in the direction of the radius TP, neither accelerates nor retards the de¬
scription of the area TPC made by that radius TP; but EL, acting on
the radius TP in a perpendicular direction, accelerates or retards the de¬
scription of the area, in proportion as it accelerates or retards the moon.

Book III.]

OF NATURAL PHILOSOPHY.

421

That acceleration of the moon, in its passage from the quadrature C to the
conjunction A, is in every moment of time as the generating accelerative

force EL, that is, as

3PK X TK
IT

Let tlie time be represented by the

mean motion of the moon, or (which comes to the same thing) by the angle
CTP, or even by the arc CP. At right angles upon CT erect CG equal
to CT; and, supposing the quadrantal arc AC to be divided into an infinite
number of equal parts P p, &c., these parts may represent the like infinite
number of the equal parts of time. Let fall pic perpendicular on CT, and
draw TG meeting with KP, kp produced in F and f; then will FK be
equal to TK, and Kk be to PK as P p to T p, that is, in a given propor¬

tion ; and therefore FK X K k, or the area F Kkf, will be as

3PK X TK
IT

that is, as EL; and compounding, the whole area GCKF will be as the
sum of all the forces EL impressed upon the moon in the whole time CP;
and therefore also as the velocity generated by that sum, that is, as the ac¬
celeration of the description of the area CTP. or as the increment of the
moment thereof. The force by which the moon may in its periodic time
CADB of 27 d . 7 h . 43' be retained revolving about the earth in rest at the
distance TP, would cause a body falling in the time CT to describe the
length |CT, and at the same time to acquire a velocity equal to that with
which the moon is moved in its orbit. This appears from Cor. 9, Prop.
IV., Book I. But since Kd, drawn perpendicular on TP, is bat a third
part of EL, and equal to the half of TP, or ML, in the octants, the force
EL in the octants, where it is greatest, will exceed the force ML in the
proportion of 3 to 2; and therefore will be to that force by which the moon
in its periodic time may be retained revolving about the earth at rest as
100 to f X 17872^, or 11915; and in the time CT will generate a ve¬
locity equal to T t£t 5 parts of the velocity of the moon; but in the time
CPA will generate a greater velocity in the proportion of CA to CT or
TP. Let the greatest force EL in the octants be represented by the area
FK X Kk, or by the rectangle |TP X Pjtf, which is equal thereto: and
the velocity which that greatest force can generate in any time CP will be
to the velocity which any other lesser force EL can generate in the same
time as the rectangle |TP X CP to the area KCGF; but the velocities
generated in the whole time CPA will be one to the other as the rectangle
}TP X CA to the triangle TCG, or as the quadrantal arc CA to the
radius TP; and therefore the latter velocity generated in the whole time
will be jtIts parts of the velocity of the moon. To this velocity of the
moon, which is proportional to the mean moment of the area (supposing
this mean moment to be represented by the number 11915), we add and
subtract the half of the other velocity; the sum 11915 -f 50, or 11965,
will represent the greatest moment of the area in the syzygy A .; and the

422

THE MATHEMATICAL PRINCIPLES

[Book III

difference 11915 — 50, or 11865, the least moment thereof in the quadra¬
tures. Therefore the areas which in equal times are described in the syzy-
gies and quadratures are one to^the other a3 11965 to 11865. And if to
the least moment 11865 we add a moment which shall be to 100, the dif¬
ference of the two former moments, as the trapezium FKCG to the triangle
TCG, or, which comes to the same thing, as the square of the sine PK to
the square of the radius TP (that is, as Pd to TP), the sum will represent
the moment of the area when the moon is in any intermediate place P.

But these things take place only in the hypothesis that the sun and the
earth are at rest, and that the synodical revolution of the moon is finished
in 27' 1 . 7 h . 43 . But since the moon’s synodical period is really 29 d . 12 h .
4 P, the increments of the moments must be enlarged in the same propor¬
tion as the time is, that is, in the proportion of 10S0853 to 1000000.
Upon which account, the whole increment, which was T j£ parts of the
mean moment, will now become ¥ parts thereof; and therefore the
moment of the area in the quadrature of the moon will be to the moment
thereof in the syzygy as 11023 — 50 to 11023 + 50; or as 10973 to
11073; and to the moment thereof, when the moon is in any intermediate
place P, as 10973 to 10973 + Pd ; that is, supposing TP = 100.

The area, therefore, which the moon, by a radius drawn to the earth,
describes in the several little equal parts of time, is nearly as the sum 0 i
the number 219,46, and the versed sine of the double distance of the moon
from the nearest quadrature, considered in a circle which hath unity for its
radius. Thus it is when the variation in the octants is in its mean quantity.
3ut if the variation there is greater or less, that versed sine must be aug¬
mented or diminished in the same proportion.

PROPOSITION XXYII. PROBLEM VIII.

From the horary motion of the moon to find its distance from the earth .

The area which the moon, by a radius drawn to the earth, describes in
every* moment of time, is as the horary motion of the moon and the square
of the distance of the moon from the earth conjunctly. And therefore the
distance of the moon from the earth is in a proportion compounded of the
subduplicate proportion of the area directly, and the subduplioate propor¬
tion of the horary motion inversely. Q,.E.I.

Cor. 1 . Hence the apparent diameter of the moon is given; for it is re¬
ciprocally as the distance of the moon from the earth. Let astronomers
try how accurately this rule agrees with the phenomena.

Cor. 2. Hence also the orbit of the moon may be more exactly defined
from the phenomena than hitherto could be done.

Book IIL!

OF NATURAL PHILOSOPHY.

423

PROPOSITION XXVIII. PROBLEM IX.

To find the diameters of the or'bit, in which , without eccentricity , the

moon woidd move.

The curvature of the orbit which a body describes, if attracted in lines
perpendicular to the orbit, is as the force of attraction directly, and the
square of the velocity inversely. I estimate the curvatures of lines com¬
pared one w ith another according to the evanescent proportion of the sines
or tangents of their angles of contact to equal radii, supposing those radii
to be infinitely diminished. Blit the attraction of the moon towards the
earth in the syzygies is the excess of its gravity towards the earth above
the force of the sun 2PK (see Pig. Prop. XXV), by which force the accel¬
erative gravity of the moon towards the sun exceeds the accelerative gravity
of the earth towards the sun, or is exceeded by it. But in the quadratures
that attraction is the sum of the gravity of the moon towards the earth,
and the sun’s force KT, by which the moon is attracted towards the earth.

And these attractions, putting N for

AT -f CT
2 "

, 178725

•, are nearly as ———-

A 1 -

2000

, 178725
and Am ~ +

1000

or as 17S725N X CT :

2000AT :

CT x N CT 3 ' AT x iY
X CT, and 178725N X AT 3 + 1000CT 3 X AT. For if the accelera¬
tive gravity of the moon towards the earth be represented by the number
17S725, the mean force ML, which in the quadratures is PT or TK, and
draws the moon towards the earth, will be 1000, and the mean force TM in
the syzygies will be 3000; from which, if we subtract the mean force ML,
there will remain 2000, the force by which the moon in the syzygies is
drawn from the earth, and which we above called 2PK. But the velocity
of the moon in the syzygies A and B is to its velocity in the quadratures
C and D as CT to AT, and the moment of the area, which the moon by
a radius drawn to the earth describes in the syzygies, to the moment of that
area described in the quadratures conjunctly; that is, as 11073CT to
10973AT. Take this ratio twice inversely, and the former ratio once di¬
rectly, and the curvature of the orb of the moon in the syzygies will be to
the curvature thereof in the quadratures as 120406729 X 17S725AT 3 X
CT 3 X N— 120406729 X 2000AT 4 X CTto 122611329 X 17S725AT*
X CT 3 x N + 122611329 X 1000CT 4 X AT, that is, as 2151969AT
X CT X N — 24081AT 3 to 2191371 AT X CT X N + 12261 CT 3 .

Because the figure of the moon’s orbit is unknown, let us, in its stead,
assume the ellipsis DBCA, in the centre of which we suppose the earth to
be situated, and the greater axis DC to lie between the quadratures as the
lesser AB between the syzygies. But since the plane of this ellipsis is re¬
volved about the earth by an angular motion, and the orbit, whose curva¬
ture we now examine, should be described in a plane void of such motion

424

THE MATHEMATICAL PRINCIPLES

[Book III

we are to consider the figure which the moon,
while it is revolved in that ellipsis, describes in
this plane, that is to say, the figure Cpa, the
several points p of which are found by assuming
any point P in the ellipsis, which may represent
the place of the inoon, and drawing T/? equal
to TP in such manner that the angle PTjt? may
he equal to the apparent motion of the sun from
the time of the last quadrature in C; or (which
comes to the same thing) that the angle CT 'p
may be to the angle CTP as the time of the
synodic revolution of the moon to the time oi
the periodic revolution thereof, or as 29' 1 .12 h . 44' to 27 d . 7 h . 43'. If, there¬
fore, in this proportion we take the angle CTa to the right angle CTA,
and make Ta of equal length with TA, we shall have a the lower and C
the upper apsis of this orbit Cpa. But, by computation, I find that the
difference betwixt the curvature of this orbit Cpa at the vertex a, and the
curvature of a circle described about the centre T with the interval TA, is
to the difference between the curvature of the ellipsis at the vertex A, and
the curvature of the same circle, in the duplicate proportion of the angle -
CTP to the angle CT p; and that the curvature of the ellipsis in A is to
the curvature of that circle in the duplicate proportion of TA to TC; and
the curvature of that circle to the curvature of a circle described about the
centre T with the interval TC as TC to TA; but that the curvature of
this last arch is to the curvature of the ellipsis in C in the duplicate pro¬
portion of TA to TC ; and that the difference betwixt the curvature of the
ellipsis in the vertex C^ and the curvature of this hist circle, is to the dif¬
ference betwixt the curvature of the figure Cpa , at the vertex C, and the
curvature of this same last circle, in the duplicate proportion of the angle
CTjt? to the angle CTP; all which proportions are easily drawn from the
sines of the angles of contact, and of the differences of those angles. But,
by comparing those proportions together, we find the curvature of the figure
Cpa at a to "be to its curvature at C as AT 3 .— tYAVoCT 3 AT to CT 3 -r
_ 1 _. 6 _ 8 _ 2 _. 4 _ AT 2 X CT; where the number T VVoVo represents the difference
of the squares of the angles CTP and CTj o, applied to the square of the
lesser angle CTP; or (which is all one) the difference of the squares of the
limes 27°. 7 h . 43', and 29 1 . 12 h . 44', applied to the square of the time27‘ 1 .
7 h . 43'.

Since, therefore, a represents the syzygy of the moon, and C its quadra¬
ture, the proportion now found must be the same with that proportion of
the curvature of the moon’s orb in the syzygies to the curvature thereof in
the quadratures, which we found above. Therefore, in order to find th<

OF NATURAL PHILOSOPHY.

425

Book I1I.J

proportion of CT to AT, let us multiply the extremes and the means, and
the terms which come out, applied to AT X CT, become 2062,79CT 4 —
2151969N X CT 3 + 36S676N X AT X CT 2 + 36342 AT 2 X CT 2 —
362047N X AT 2 X CT + 2191371N X AT 3 + 4051,4AT 4 = 0.
Now if for the half sum N of the terms AT and CT we put 1, and x for
their half difference, then CT will be = 1 + x, and AT = 1 — x. And
substituting those values in the equation, after resolving thereof, we shall
find x — 0,00719; and from thence the semi-diameter CT = 1,00719, and
the semi-diameter AT == 0,99281, which numbers are nearly as 70^, and
693 V- Therefore the moon’s distance from the earth in the syzygies is to
its distance in the quadratures (setting aside the consideration of eccentrici¬
ty) as 693 V to 70 t j t ; or, in round numbers, as 69 to 70.

PROPOSITION XXIX. PROBLEM X.

To find the variation of the moon.

This inequality is owing partly to the elliptic figure of the moon’s orbit,
partly to the inequality of the moments of the area which the moon by a
radius drawn to the earth describes. If the moon P revolved in the ellipsis
DBCA about the earth quiescent in the centre of the ellipsis, and by the
radius TP, drawn to the earth, described the area CTP, proportional to
the time of description ; and the greatest semi-diameter CT of the ellipsis
was to the least TA as 70 to 69; the tangent of the angle CTP would be
to the tangent of the angle of the mean motion, computed from the quad¬
rature C, as the semi-diameter TA of the ellipsis to its semi-diameter TC,
or as 69 to 70. But the description of the area CTP, as the moon advan¬
ces from the quadrature to the syzygy, ought to be in such manner accel¬
erated, that the moment of the area in the moon’s syzygy may be to the
moment thereof in its quadrature as 11073 to 10973; and that the excess
of the moment in any intermediate place P above the moment in the quad¬
rature may be as the square of the sine of the angle CTP ; which we may
effect with accuracy enough, if we diminish the tangent of the angle CTP
in the subduplicate proportion of the number 10973 to the number 11073,
that is, in proportion of the number 6S,6877 to the number 69. Upon
which account the tangent of the angle CTP will now be to the tangent
of the mean motion as 68 , 6 S77 to 70; and the angle CTP in the octants,
where the mean motion is 45°, will be found 44° 27' 28", which sub¬
tracted from 45°, the angle of the mean motion, leaves the greatest varia¬
tion 32' 32". Thus it would be, if the moon, in passing from the quad¬
rature to the syzygy, described an angle CTA of 90 degrees only. But
because of the motion of the earth, by which the sun is apparently trans¬
ferred in CGnsequentia, the moon, before it overtakes the sun, describes an
angle CTer, greater than a right angle, in the proportion of the time of the
synodic revolution of the moon to the time of its periodic revolution, thal

126

THE MATHEMATICAL PRINCIPLES

[Book III

is, in the proportion of 29' 1 . 12 h . 44' to 27 A . 7". 43'. Whence it comes tc
pass that all the angles about the centre T are dilated in the same pro¬
portion • and the greatest variation, which otherwise would be but 32'
32", now augmented in the said proportion, becomes 35' 10".

And this is its magnitude in the mean distance of the sun from the
earth, neglecting the differences which may arise from the curvature of
the orbis magnus , and the stronger action of the sun upon the moon when
horned and new, than when gibbous and full. In other distances of the
sun from the earth, the greatest variation is in a proportion compounded
of the duplicate proportion of the time of the synodic revolution of the
moon (the time of the year being given) directly, and the triplicate pro¬
portion of the distance of the sun from the earth inversely. And, there¬
fore, in the apogee of the sun, the greatest variation is 33' 14", and in its
perigee 3 7' 11", if the eccentricity of the sun is to the transverse semi-di¬
ameter of the orbis magnus as 16}£ to 1000.

Hitherto we have investigated the variation in an orb not eccentric, in
which, to wit, the moon in its octants is always in its mean distance from
the earth. If the moon, on account of its eccentricity, is more or less re¬
moved from the earth than if placed in this orb, the variation may be
something greater, or something less, than according to this rule. But I
leave the excess or defect to the determination of astronomers from the
phenomena.

PROPOSITION XXX. PROBLEM XI.

To find the horary motion of the nodes of the moon in a circular orbit.

Let S represent the sun, T the earth, P the moon, NP//- the orbit of the
moon, Nj on the orthographic projection of the orbit upon the plane of th*
ecliptic: N. n the nodes, wTNm the line of the nodes produced indeii«

7

Book III.]

OF NATURAL PHILOSOPHY.

427

nitely; PI, PK perpendiculars upon the lines ST, Q,^; Pj o a perpendicu¬
lar upon the plane of the ecliptic; A, B the moon’s syzygies in the plane
of the ecliptic; AZ a perpendicular let fall upon N 7 /., the line of the
nodes; Q,, q the quadratures of the moon in the plane of the ecliptic, and
pK a perpendicular on the line Q ,q lying between the quadratures. The
force of the sun to disturb the motion of the moon (by Prop. XXV) is
tw'ofold, one proportional to the line LM, the other to the line MT, in the
scheme of that Proposition; and the moon by the former force is drawn
towards the earth, by the latter towards the sun, in a direction parallel to
the right line ST joining the earth and the sun. The former force LM
acts in the direction of the plane of the moon’s orbit, and therefore makes
no change upon the situation thereof, and is upon that account to be neg¬
lected ; the latter force MT, by which the plane of the moon’s orbit is dis¬
turbed, is the same with the force 3PK or 3IT. And this force (by Prop.
XXV) is to the force by which the moon may, in its periodic time, be uni¬
formly revolved in a circle about the earth at rest, as 3IT to the radius of
the circle multiplied by the number 178,725, or as IT to the radius there¬
of multiplied by 59,575. But in this calculus, and all that follows, I
consider all the lines drawn froni the moon to the sun as parallel to the
line which joins the earth and the sun; because what inclination there is
almost as much diminishes all effects in some cases as it augments them
in others: and we are now inquiring after the mean motions of the nodes,
neglecting such niceties as are of no moment, and would only serve to ren¬
der the calculus more perplexed.

Now suppose PM to represent an arc which the moon describes in the
least moment of time, and ML a little line, the half of which the moon,
by the impulse of the said force SIT, would describe in the same time; and
joining PL, MP, let them be produced to m and l, where they cut the plane
of the ecliptic, and upon Tm let fall the perpendicular PH. Now, since
the right line ML is parallel to the plane of the ecliptic, and therefore can
never meet with the right line 7nl which lies in that plane, and yet both
those right lines lie in one common plane LMPm/, they will be parallel,
and upon that account the triangles LMP, ImP will be similar. And
seeing MPm lies in the plane of the orbit, in which the moon did move
while in the place P, the point m will fall upon the line N?/, which passes
through the nodes N, w, of that orbit. And because the force by which the
half of the little line LM is generated, if the whole had been together, and
it once impressed in the point P, would hav^ generated that whole line,
and caused the moon to move in the arc whose chord is LP; t at is to say,
would have transferred the moon from the plane MPraT into the plane
LP/T; therefore the angular motion of the nodes generated by that force
will be equal to the angle rnTTl. But ml is to mP as ML to MP; and
since MP, because of the time given, i3 also given, ml will be as the rcctan-

428

THE MATHEMATICAL PRINCIPLES

[Book III.

gle ML X mV, that is, as the rectangle IT X mV. And if T ml is a right

Till IT X P/71

anode, the angle mTl will be as and therefore as —^ -- that is (be-

n 7 ° 1 m Lm '

cause T m and mV, TP and PH are proportional), as

IT X PH ;

"tp

and, there¬

fore, because TP is given, as IT X PH. But if the angle T ml or STN
is oblique, the angle mdVl will be yet less, in proportion of the sine of the
angle STN to the radius, or AZ to AT. And therefore the velocity of
the nodes is as IT X PH X AZ, or as the solid content of the sines of the
three angles TPI, PTN, and STN.

If these are right angles, as happens when the nodes are in the quadra¬
tures, and the moon in the syzygy, the little line ml will be removed to
an infinite distance, and the angle m r Vl will become equal to the angle
mVl. But in this case the angle mVl is to the angle PTM, which the
moon in the same time by its apparent motion describes about the earth,
as 1 to 59,575. For the angle mVl is equal to the angle LPM, that is, to
the angle of the moon’s deflexion from a rectilinear path; which angle, if
the gravity of the moon should have then ceased, the said force of the sun
3IT would by itself have generated in that given time; and the angle
PTM is equal to the angle of the moon’s deflexion from a rectilinear path;
which angle, if the force of the sun 3IT should have then ceased, the force
alone by which the moon is retained in its orbit would have generated in
the same time. And these forces (as we have above shewn) are the one to
the other as 1 to 59,575. Since, therefore, the mean horary motion of the
moon (in respect of the fixed stars) is 32' 56" 2 7'" 12P V . the horary motion
of the node in this case will be 33" 10"' 33 lv . 12 v . But in other cases the
horary motion will be to 33" 10'" 33 iv . 12 v . as the solid content of the sines
of the three angles TPI, PTN, and STN (or of the distances of the moon
from the quadrature, of the moon from the node, and of the node from the
sun) to the cube of the radius. And as often as the sine of any angle is
changed from positive to negative, and from negative to positive, so often
must the regressive be changed into a progressive, and the progressive into
a regressive motion. Whence it comes to pass that the nodes are pro¬
gressive as often as the moon happens to be placed between either quadra¬
ture, and the node nearest to that quadrature. In other cases they are
regressive, and by the excess of the regress above the progress, they are
monthly transferred in antecedentia.

Cor. 1. Hence if from P and M, the extreme points of a least arc PM,
on the line Qq joining the quadratures we let fail the perpendiculars PK
MAr, and produce the same till they cut the line of the nodes N n in D ana
d, the horary motion of the nodes will be as the area MPDg?, and the
square of the line AZ conjunctly. For let PK, PH, and AZ, be the three
said sines, viz., PK the sine of the distance of the moon from the quadra-

Book III.]

OF NATURAL PHILOSOPHY.

421*

ture, PH the sine of the distance of the moon from the node, and AZ the
sine of the distance of the node from the sun; and the velocity of the node
will be as the solid content of PK X PH X AZ. But PT is to PK as
PM to KA*; and, therefore, because PT and PM are given, K k will be as
PK. Likewise AT is to PD as AZ to PH, and therefore PH is as the
rectangle PD X AZ; and, by compounding those proportions, PK X PH
is as the solid content KA X PD X AZ, and PK X PH X AZ as KA*
X PD X AZ 2 ; that is, as the area PDc/M and AZ a conjunctly. Q.E.D.

Cor. 2. In any given position of the nodes their mean horary motion is
half their horary motion in the moon’s syzygies; and therefore is to 16"
35"' 16 iv . 36 v . as the square of the sine of the distance of the nodes from
the syzygies to the square of the radius, or as AZ 2 to AT 2 . For if the
moon, by an uniform motion, describes the semi-circle QA</, the sum of all
the areas PDt/M, during the time of the moon’s passage from Q, to M, will
make up the area QM</E, terminating at the tangent Q,E of the circle;
and by the time that the moon has arrived at the point n, that sum will
make up the whole area EQAra described by the line PD: but when the
moon proceeds from n to q, the line PD will fall without the circle, and
describe the area nqe, terminating at the tangent qe of the circle, which
area, because the nodes were before regressive, but are now progressive,
must be subducted from the former area, and, being itself equal to the area
Q,EN, will leave the semi-circle NQA n. While, therefore, the moon de¬
scribes a semi-circle, the sum of all the areas PDrfM will be the area of
that semi-circle; and while the moon describes a complete circle, the sum
of those areas will be the area of the whole circle. But the area PDcfM,
when the moon is in the syzygies, is the rectangle of the arc PM into the
radius PT; and the sum of all the areas, every one equal to this area, in
the time that the moon describes a complete circle, is the rectangle of the
whole circumference into the radius of the circle; and this rectangle, being
double the area of the circle, will be double the quantity of the former sum

130

THE MATHEMATICAL PRINCIPLES

[Book 111

If, therefore, the nodes went on with that velocity uniformly continued
which they acquire in the moon’s syzygies, they would describe a space
double of that which they describe in fact; and, therefore, the mean motion,
by which, if Uniformly continued, they would describe the same space with
that which they do in fact describe by an unequal motion, is but one-half
of that motion which they are possessed of in the moon's syzygies. Where¬
fore since their greatest horary motion, if the nodes are in the quadratures,
is 33" 10'" 33 iv . 1 2 V . their mean horary motion in this case will be 16"
35"' 16 iv . 36 v . And seeing the horary motion of the nodes is every where
as AZ 2 and the area PDrfM conjunctly, and, therefore, in the moon’s
syzygies, the horary motion of the nodes is as AZ 2 and the area PDc?M
conjunctly, that is (because the area PDdM described in the syzygies is
given), as AZ 3 , therefore the mean motion also will be as AZ 2 ; and, there¬
fore, when the nodes are without the quadratures, this motion will be to
16" 35'" 16 lV . 36 v . as AZ 2 to AT 2 . Q.E.D.

PROPOSITION XXXI. PROBLEM XII.

To find the horary motion of the tiodes of the moon in an elliptic orbit
Let Qjpmaq represent an ellipsis described with the greater axis Q, q, am
the lesser axis ab\ QA^B a circle circumscribed ; T the earth in the com¬
mon centre of both; S the sun; p the moon moving in this ellipsis; and

Book I1I.J

OF NATURAL PHILOSOPHY.

431

pm an arc which it describes in the least, moment of time; N'and n the
nodes joined by the line Nn.; pK and mk perpendiculars upon the axis Q,q,
produced both ways till they meet the circle in P and M, and the line of
the nodes in D and d. And if the moon, by a radius drawn to the earth,
describes an area proportional to the time of description , the horary motion
of the node in the ellipsis will be as the area pDdm and AZ 2 conjunctly.

For let PF touch the circle in P, and produced meet TN in F; and pj
touch the ellipsis in p ) and produced meet the same TN inland both
tangents concur in the axis TQ at Y. And let ML represent the space
which the moon, by the impulse of the above-mentioned force 3IT or 3PK,
would describe with a transverse motion, in the meantime while revolving
in the circle it describes the arc PM; and ml denote the space which the
moon revolving in the ellipsis would describe in the same time by the im¬
pulse of the same force 31T cr 3PK ; and‘let LP and Ip be produced till
they meet the plane of the ecliptic in G and g-, and FG and fg be joined,
of which FG produced may cut pf pg, and TQ,, in c , e, and R respect¬
ively ; and fg produced may cut TQ in r. Because the force 3IT or 3PK
in the circle is to the force 3IT or 3jt?K in the ellipsis as PK to pK, or
as AT to </T, the space MI < generated by the former force will be to the
space ml generated by the latter as PK to y?K; that is, because of the
similar figures PYK p and FYRc, as FR to cR. But (because of the
similar triangles PLM, PGF) ML is to FG as PL to PG, that is (on ac¬
count of the parallels L k, PK, GR), as pi to joe, that is (because of the
similar triangles plm, epe), as lm to ce; and inversely as LM is to /m, or
as FR is to cR, so is FG to ce. And therefore if fg was to ce as fy to
cY, that is, as fr to cR (that is, as fr to FR and FR to cR conjunctly,
that is, as /T to FT, and FG to ce conjunctly), because the ratio of FG
to ce, expunged on both sides, leaves the ratios fg to FG and fT to FT,
fg would be to FG as fT to FT; and, therefore, the angles which FG
and fg would subtend at the earth T would be equal to each other. But
these angles (by what we have shewn in the preceding Proposition) are the
motions of the nodes, while the moon describes in the circle the arc PM,

in the ellipsis the arc pm ; and therefore the motions of the nodes in the
circle and in the ellipsis would be equal to each other. Thus, I say, it

ceX fY

would be, if fg was to ce as fY to cY, that is, if fg was equal to——-•

But because of the similar triangles fgp, cep , fg is to ce as fp to cp ; and
therefore fg is equal to——; and therefore the angle which fg sub¬
tends in fact is to the former angle which FG subtends, that is to say, the
motion of the nodes in the ellipsis is to the motion of the same in the

circle as this fg ——to the former fg or—^-, that is, as fp X

THE MATHEMATICAL PRINCIPLES

[Book 111.

432

cY to / Y X cp, or as fp to fY, and cY to cp ; that is, if ph parallel to
TN meet FP in h, as F h to FY and F Y to FP ; that is, as F h to FP
or T>p to DP, and therefore as the area D prnd to the area DPMd. And,
therefore, seeing (by Corol. 1, Prop. XXX) the latter area and AZ 2 con-
jnnctly are proportional to the horary motion of the nodes in the circle,
the former area and AZ 2 conjunctly will be proportional to the horary
motion of the nodes in the ellipsis. Q.E.D.

Cor. Since, therefore, in any given position of the nodes, the sum of all
the areas pDdrn , in the time while the moon is carried from the quadra¬
ture to any place m, is the area mpQEd terminated at the tangent of the
ellipsis QE ; and the sum of all those areas, in one entire revolution, is
the area of the whole ellipsis; the mean motion of the nodes in the ellip¬
sis will be to the mean motion of the nodes in the circle as the ellipsis to
the circle; that is, as T a to TA, or 69 to 70. And, therefore, since (by
Cwrol 2, Prop. XXX) the mean horary motion of the nodes in the circle
is to 16" 35'" 16 iv . 36 v . as AZ 2 to AT 2 , if we take the angle 16" 21'"
3 iv . 30 v . to the angle 16" 35'" 16 iv . 36 v . as 69 to 70, the mean horary mo¬
tion of the nodes in the ellipsis will be to 16" 21'" 3 iv . 30 v . as AZ 2 to
AT 2 ; that is, as the square of the sine of the distance of the node from
the sun to the square of the radius.

But the moon, by a radius drawn to the earth, describes the area in the
syzygies with a greater velocity than it does that in the quadratures, and
upon that account the time is contracted in the syzygies, and prolonged in
the quadratures; and together with the time the motion of the nodes is
likewise augmented or diminished. But the moment of the area in the
quadrature of the moon was to the moment thereof in the syzygies as
10973 to 11073 ; and therefore the mean moment in the octants is to the
excess in the syzygies. and to the defect in the quadratures, as 11023, the
half sum of those numbers, to their half difference' 50. Wherefore since
the time of the moon in the several little equal .parts of its orbit is recip¬
rocally as its velocity, the mean time in the octants will be to the excess
of the time in the quadratures, and to the defect of the time in the syzy¬
gies arising from this cause, nearly as 11023 to 50. But, reckoning from
the quadratures to the syzygies, I find that the excess of the moments of
the area, in the several places above the least moment in the quadratures,
is nearly as the square of the sine of the moon’s distance from the quad¬
ratures : and therefore the difference betwixt the moment in any place,
and the mean moment in the octants, is as the difference betwixt the square
of the sine of the moon’s distance from the quadratures, and the square
of the sine of 45 degrees, or half the square of the radius; and the in¬
crement of the time in the several places between the octants and quad¬
ratures, and the decrement thereof between the octants and syzygies, is in
the same proportion. But the motion of the nodes, while the moon de¬
scribes the several little equal parts of its orbit, is accelerated or retarded

OF NATURAL PHILOSOPHY.

433

Book III.]

in the duplicate proportion of the time; for that motion, while the moon
describes PM, is (cceteris paribus) as ML, and ML is in the duplicate
proportion of the time. Wherefore the motion of the nodes in the syzy-
gjes, in the time while the moon describes given little parts of its orbit,
is diminished in the duplicate proportion of the number 11073 to the num¬
ber 11023: and the decrement is to the remaining motion as 100 to
10973 ; but to the whole motion as 100 to 11073 nearly. But the decre¬
ment in the places between the octants and syzygies, and the increment in
the places between the octants and quadratures, is to this decrement nearly
as the whole motion in these places to the whole motion in the syzygies,
and the difference betwixt the square of the sine of the moon’s distance
from the quadrature, and the half square of the radius, to the half square
of the radius conjunctly. Wherefore, if the nodes are in the quadratures,
and we take two places, one on one side, one on the other, equally distant
from the octant and other two distant by the same interval, one from the
syzvgy, the other from the quadrature, and from the decrements of the
motions in the two places between the syzygy and octant we subtract the
increments of the motions in the two other places between the octant and
the quadrature, the remaining decrement will be equal to the decre¬
ment in the syzygy, as will easily appear by computation; and therefore
the mean decrement, which ought to be subducted from the mean motion
of the nodes, is the fourth part of the decrement in the syzygy. The
whole horary motion of the nodes in the syzygies (when the moon by a ra¬
dius drawn to the earth was supposed to describe an area proportional to
the time) was 32" 42"' ? iv . And we have shewn that the decrement of
the motion of the nodes, in the time while the moon, now moving with
greater velocity, describes the same space, was to this motion as 100 to
11073; and therefore this decrement is 17'" 43 iv . ll v . The fourth part
of which 4'" 25 iv . 48 v . subtracted from the mean horary motion above
found, 16" 21'" 3 iv . 30 v .-leaves 16" 16'" 37 iv . 42 v . their correct mean ho¬
rary motion.

If the nodes are without the quadratures, and two places are considered,
one on one side, one on the other, equally distant from the syzygies, the
sum of the motions of the nodes, when the moon is in those places, will be
to the sum of their motions, when the moon is in the same places and the
nodes in the quadratures, as AZ 2 to AT 2 . And the decrements of the
motions arising from the causes but now explained will be mutually as
the motions themselves, and therefore the remaining motions will be mu¬
tually betwixt themselves as AZ 2 to AT 2 ; and the mean motions will be
as the remaining motions. And, therefore, in any given position of the
nodes, their correct mean horary motion is to 16" 16'" 37 iv . 42 v . as AZ 2
to AT 2 ; that is, as the square of the sine of the distance of the nodes
from the syzygies to the square of the radius.

- 28

134

THE MATHEMATICAL PRINCIPLES

[Book IIL

PROPOSITION XXXII. PROBLEM XIII.

To find the mean motion of the nodes of the moon.

The yearly mean motion is the sum of all the mean horary motions
throughout the course of the year. Suppose that the node is in N, and
that, after every hour is elapsed, it is drawn back again to its former
place; so that, notwithstanding its proper motion, it may constantly re¬
main in the same situation with respect to the .fixed stars; while in the
mean time the sun S, by the motion of the earth, is seen to leave the node,

and to proceed till it completes its appa¬
rent annual course by an uniform motion.
Let Aa represent a given least arc, which
the right line TS always drawn to the
sun, by its intersection with the circle
NA?/, describes in the least given moment
of time; and the mean horary motion
(from what we have above shewn) will be
as AZ 2 , that is (because AZ and ZY are
proportional), as the rectangle of AZ into ZY. that is, as the area
AZY a ; and the sum of all the mean horary motions from the beginning
will be as the sum of all the areas nYZA, that is, as the area NAZ. But
the greatest AZY a is equal to the rectangle of the arc A a into the radius
of the circle; and therefore the sum of all these rectangles in the whole
circle will be to the like sum of all the greatest rectangles as the area of
the whole circle to the rectangle of the whole circumference into the ra¬
dius, that is, as 1 to 2. But the horary motion corresponding to that
greatest rectangle was 16" 16"' 37 iv . 42 v . and this motion in the complete
course of the sidereal year, 365 d . 6\ 9' } amounts to 39° 38' 7" 50"', and
therefore the half thereof, 19° 49' 3" 55"', is the mean motion of the
nodes corresponding to the whole circle. And the motion of the nodes,
in the time while the sun is carried from N to A, is to 19° 49' 3" 5o'" as
the area NAZ to the whole circle.

Thus it would be if the node was after every hour drawn back again to
its former place, that so, after a complete revolution, the sun at the years
end would be found again in the same node which it had left when the
year begun. But, because of the motion of the node in the mean time, the
sun must needs meet the node sooner; and now it remains that we compute
the abbreviation of the time Since, then, the sun, in the course of the
year, travels 360 degrees, and the node in the same time by its greatest
motion would be carried 39 J 38' 7" 50'", or 39,6355 degrees; and the mean
motion of the node in any place N is to its mean motion in its quadratures
as AZ 2 to AT a • the motion of the sun will be to the motion of the nods

OF NATURAL PHILOSOPHY.

43 b

Book III.]

in N as 360AT 2 to 39,6355AZ 2 : that is, as 9,0S27646AT 2 to AZ 2 .
Wherefore if we suppose the circumference NA/t of the whole circle to be
divided into little equal parts, such as Art, the time in which the sun would
describe the little arc Aa, if the circle wa3 quiescent, will be to the time of
which it would describe the same arc, supposing the circle together with
the nodes to be revolved about the centre T, reciprocally as 9,0827646AT 2
to 9,0827646AT 2 -f AZ 2 ; for the time is reciprocally as the velocity
with which the little arc is described, and this velocity is the sum of the
velocities of both sun and node. If, therefore, the sector NTA represent
the time in which the sun by itself, without the motion of the node, would
describe the arc NA, and the indefinitely small part ATa of the sector
represent the little moment of the time in which it would describe the least
arc A a; and (letting fall «Y perpendicular upon N//) if in AZ we take
d Z of such length that the rectangle of dZ into ZY may be to the least
part ATa of the sector as AZ 2 to 9,0827646AT 2 4- AZ 2 , that is to
say, that dZ may be to \AZ* as AT 2 to 9,0S27646AT 2 -f- AZ 2 ; the
rectangle of d Z into ZY will represent the decrement of the time arising
from the motion of the node, while the arc A a is described; and if the
curve N dGn is the locus where the point d is always found, the curvilinear
area NrfZ will be as the whole decrement of time while the whole arcNA
is described; and, therefore, the excess of the sector NAT above the area
Nc/Z will be as the whole time. But because the motion of the node in a
less time is less in proportion of the time, the area AdYZ must also be di¬
minished in the same proportion ; which may be done by taking in AZ the
line'eZ of such length, that it may be to the length of AZ as AZ 2 to
9,0S27646AT 2 -f- AZ 2 ; for so the rectangle of eZ into ZY will be to
the area AZY a as the decrement of the time in which the arc Aa is de¬
scribed to the whole time in which it would have been described, if the
node had been quiescent; and, therefore, that rectangle will be as the de¬
crement of the motion of the node. And if the curve NeF?i is the locus of
the point e, the whole area NeZ, which is the sum of all the decrements of
that motion, will be as the whole decrement thereof during the time in
which the arc AN is described ; and the remaining area NAe will be as the
remaining motion, which is the true motion of the node, during the time
in which the whole arc NA is described by the joint motions of both sun
and node. Now the area of the semi-circle is to the area of the figure
NeFn found by the method of infinite series nearly as 793 to d \ But the
motion corresponding or proportional to the whole circle was 19° 49' 3"
55'"; and therefore the motion corresponding to double the figure NeFn
is 1° 29' 58" 2'", which taken from the former motion leaves 18° 19' 5"
53"', the whole motion of the node witn respect to the fixed stars in the
interval between two of its conjunctions with the sun; and this motion sub¬
ducted from the annual motion of the sun 360% leaves 341° 40' 54" 7'",

436

THE MATHEMATICAL PRINCIPLES

HBook HI.

the motion of the sun in the interval between the same conjunctions. But
ns this motion is to the annual motion 360°, so is the motion of the node
but just now found IS 0 19' 5" 53"' to its annual motion, which will there¬
fore be 19° 18' 1" 23'"; and this is the mean motion of the nodes in the
sidereal year. By astronomical tables, it is 19° 21' 21" 50'". The dif¬
ference is less than part of the whole motion, and seeips to arise from
the eccentricity of the moon’s orbit, and its inclination to the plane of the
ecliptic. By the eccentricity of this orbit the motion of the nodes is too
much accelerated; and, on the other hand, by the inclination of the orbit,
the motion of the nodes is something retarded, and reduced to its just
velocity.

PROPOSITION XXXIII. PROBLEM XIV.

To find the true motion of the nodes of the moon.

In the time which is as the area
NT A—N</Z (in the preceding Fig.)
. A that motion is as the area NAe, and
is thence given ; but because the cal¬
culus is too difficult, it will be better
to use the following construction of
the Problem. About the centre C,
with any interval CD, describe the circle BEFD; produce DC to A so as
AB may be to AC as the mean motion to half the mean true motion when
the nodes are in their quadratures (that is, as 19° 18' 1" 23'" to 19^ 49' 3"
55'" ; and therefore BC to AC as the difference of those motions 0° ol' 2"
32'" to the latter motion 19° 49' 3" 55'", that is, as 1 to 38 T \). Then
through the point D draw the indefinite line G g y touching the circle in
D ; and if we take the angle BCE, or BCF, equal to the double distance
of the sun from the place of the node, as found by the mean motion, and
drawing AE or AF cutting the perpendicular DG in G, we take another
angle which shall he to the whole motion of the node in the interval be¬
tween its syzygies (that is, to 9° 11' 3") as the tangent DG to the whole
circumference of the circle BED, and add this last angle (for which the
angle DAG may be used) to the mean motion of the nodes, while they are
passing from the quadratures to the syzygies, and subtract it from their
mean motion while they are passing from the syzygies to the quadratures,
we shall have their true motion; for the true motion so found will nearly
agree with the true motion which comes out from assuming the times as
the area NTA—NrfZ, and the motion of the node as the area NAe; as
whoever will please to examine and make the computations will find: and
this is the semi-menstrual equation of the motion of the nodes. But there
is also a menstrual equation, but which is by no means necessary for find-

OF NATURAL PHILOSOPHY.

437

Book III.]

ing of the moon’s latitude; for since the variation of the inclination of the
moon’s orbit to the plane of the ecliptic is liable to a twofold inequality,
the one semi-inenstrual, the other menstrual, the menstrual inequality of
this variation , and the menstrual equation of the nodes, so moderate and
correct each other, that in computing the latitude of the moon both may
be neglected.

Cor. From this and the preceding Prop, it appears that the nodes are
quiescent in their syzygies, but regressive in their quadratures, by an
hourly motion of 16" 19'" 26 IV .: and that the equation of the motion of
the nodes in the octants is 1° 30'; all which exactly agree with the phe¬
nomena of the heavens.

SCHOLIUM.

Mr. Machin , Astron., Prof. Gresh., and Dr. Henry Pemberton , sepa¬
rately found out the motion of the nodes by a different method. Mention
has been made of this method in another place. Their several papers, both
of which I have seen, contained two Propositions, and exactly agreed with
each other in both of them. Mr. Machines paper coming first to my hands,
l shall here insert it.

OF THE MOTION OF THE MOON’S NODES.

'■ PROPOSITION I.

• The wean motion of the snn from the node is defined by a geometric
mean proportional between the mean motion of the sun and that mean
motion with which the sun recedes with the greatest swiftness from the
node in the quadratures.

“ Let T be the earth’s place, Nrc the line of the moon’s nodes at any
given time, KTM a perpendicular thereto, TA a right line revolving
about the centre with the same angular velocity with which the sun and
the node recede from one another, in such sort that the angle between the
quiescent right line Nw. and the revolving line TA may be always equal
to the distance of the places of the sun and node. Now if any right line
TK be divided into parts TS and SK, and those parts be taken as the
mean horary motion of the sun to the mean horary motion of the node in
the quadratures, and there be taken the right line TH, a mean propor¬
tional between the part TS and the whole TK, this right line will be pro¬
portional to the sun’s mean motion from the node.

“ For let there be described the circle NKwM from the centre T and
with the radius TK, and about the same centre, with the semi-axis TH

438

THE MATHEMATICAL PRINCIPLES

[Book Ilf

N

and TN, let there be described an ellipsis NHwL; and in the time in
which the sun recedes from the node through the arc Na, if there be drawn
the right line T ba, the area of the sector NTa will be the exponent of the
sutn of the motions of the sun and node in the same time. Let, there¬
fore, the extremely small arc «A be that which the right line T ba, revolv¬
ing according to the aforesaid law, will uniformly describe in a given
particle of time, and the extremely small sector TAa will be as the sum
of the velocities with which the sun and node are carried two different
ways in that time. Now the sun’s velocity is almost uniform, its ine¬
quality being so small as scarcely to produce the least inequality in the
mean motion of the nodes. The other part of this sum, namely, the mean
quantity of the velocity of the node, is increased in the recess from the
gyzygies in a duplicate ratio of the sine of its distance from the sun (by
Cor. Prop. XXXI, of this Book), and, being greatest in its quadratures
with the sun in K, is in the same ratio to the sun’s velocity as SK to TS,
that is, as (the difference of the squares of TK and TH, or) the rectangle
KHM to TH 2 . But the ellipsis NBH divides the sector ATor, the expo¬
nent of the sum of these two velocities, into two parts AB ba and BT6,
proportional to the velocities. For produce BT to the circle in ft and
from the point B let fall upon the greater axis the perpendicular BG,
which being produced both ways may meet the circle in the points F and
f; and because the space AB6a is to the sector TB6 as the rectangle AB p
to BT 2 (that rectangle being equal to the difference of the squares of TA
and TB, because the right line Ad is equally cut in T, and unequally in
B), therefore when the space AB ba is the greatest of all in K, this ratio
will be the same as the ratio of the rectangle KHM to HT*. But the
greatest mean velocity of the node was shewn above to be in that very

Book IIL]

OF NATURAL PHILOSOPHY.

439

ratio to the velocity of the sun; and therefore in the quadratures the sec¬
tor ATtt is divided into parts proportional to the velocities. And because
the rectangle KHM is to HT 2 as FB/ to BG 2 , and the rectangle AB/3 is'
equal to the rectangle FB/, therefore the little area AB ba, where it is
greatest, is to the remaining sector TB6 as the rectangle AB/3 to BG 2
But the ratio of these little areas always was as the rectangle AB/3 to
BT 2 ; and therefore the little area AB6« in the place A is less than its
correspondent little area in the quadratures in the duplicate ratio cf BG
to BT, that is, in the duplicate ratio of the sine of the sun’s distance
from the node. And therefore the sum of all the little areas A Bba, to
wit, the space ABN, will be as the motion of the node in the time in
which the sun hath been going over the arc NA since he left the node;
and the remaining space, namely, the elliptic sector NTB, will be as < lie
sun’s mean motion in the same time. And because the mean annual mo¬
tion of the node is that motion which it perforins in the time that the sun
completes one period of its course, the mean motion of the node from the
sun will be to the mean motion of the sun itself as the area of the circle
to the area of the ellipsis; that is, as the right line TK to the right line
TH, which is a mean proportional between TK and TS; or, which comes
to the same as the mean proportional TH to the right line TS.

PROPOSITION II.

M The mean motion of the moon’s nodes being- given , to find their true

motion.

“ Let the angle A be the distance of the sun from the mean place of the
node, or the sun’s mean motion from the node. Then if we take the angle
B, whose tangent is to the tangent of the angle A as TH to TK, that ia,

N

M

J3

440 THE MATHEMATICAL PRINCIPLES [BOOK JIJ.

in the sub-duplicatc ratio of the mean horary motion of the sun to the
mean horary motion of the sun from the node, when the node is in the
quadrature, that angle B will be the distance of the sun from the node’s
true place. For join FT, and, by the demonstration of the last Propor¬
tion, the angle FTN will be the distance of the sun from the mean place
of the node, and the angle ATN the distance from the true place, and the
tangents of these angles are between themselves as TK to TH.

“ Cor. Hence the angle FTA is the equation of the moon’s nodes ; and
the sine of this angle, where it is greatest in the octants, is to the radius
as KH to TK + TH. But the sine of this equation in any other place
A is to the greatest sine as the sine of the sums of the angles FTN +
ATN to the radius ; that is, nearly as the sine of double the distance of
the sun from the mean place of the node (namely, 2FTN) to the radius.

“SCHOLIUM.

“ If the mean horary motion of the nodes in the quadratures be 16"
16'" 37 iv . 42 v . that is, in a whole sidereal year, 39° 38' 7" 50'", TH will
be to TK in the subduplicate ratio of the number 9,0827646 to the num¬
ber 10,0827646, that is, as 18,6524761 to 19,6524761. And, therefore.
TH is to HK as 18,6524761 to 1 ; that is, as the motion of the sun in a
sidereal year to the mean motion of the node 19° 18' 1" 23f ,,/ .

“ But if the mean motion of the moon’s nodes in 20 Julian years is
386° 50' 15", as is collected from the observations made use of in the
theory of the moon, the mean motion of the nodes in one sidereal year will
be 19° 20' 31" 58'". and TH will be to HK as 360° to 19° 20' 31"
58"'; that is, as 18,61214 to 1: and from hence the mean horary motion
of the nodes in the quadratures will come out 16" 18'" 48 iv . And the
greatest equation of the nodes in the octants will be 1° 29' 57".”

PROPOSITION XXXIV. PROBLEM XV.

To find the horary variation of the inclination of the moon’s orbit to the

plane of the ecliptic.

Let A and a represent the syzygies; Q, and q the quadratures: N and
n the nodes; P the place of the moon in its orbit; p the orthographic
projection of that place upon the plane of the ecliptic; and mTl the mo-
mentaneous motion of the nodes as above. If upon T m we let fall the
perpendicular PC, and joining pG we produce it till it meet T/ in g, and
join also P g, the angle PGjo will be the inclination of the moon's orbit to
the plane of the ecliptic when the moon is in P ; and the angle P gp will
be the inclination of the same after a small moment of time is elapsed;
and therefore the angle GPg* will be the momentaneous variation of the
inclination. But this angle GPg- is to the angle GTg- as TG to PG and
P p to PG conjunctly. And, therefore, if for the moment of time we as-

ill

r?i /

Btune an hour, since the angle GT g (by Prop. XXX) is to the angle 33 ;
10'" 33 iv . as IT X PG X AZ to AT 3 , the angle GPg- (or the horary va¬
riation of the inclination) will be to the angle 33" 10'" 33 lv . as IT X AZ

X TG x Hr, to AT 3 . Q.E.I.

I Vjr

And thus it would be if the moon was uniformly revolved in a circular
orbit. But if the orbit is elliptical, the mean motion of the nodes will
be diminished in proportion of the lesser axis to the greater, as we have
shewn above; and the variation of the inclination will be also diminished
in the same proportion.

Cor. 1. Upon N a erect the perpendicular TF, and let joM be the horary
motion of the moon in the plane of the ecliptic; upon QT let fall the
perpendiculars pK, M k, and produce them till they meet TF in H and A;
then IT will be to AT as K k to Mp; and TG to Hjt? as TZ to AT; and,

Kk X Ho X TZ

therefore, IT X TG will be equal to--, that is, equal to

TZ

the area HjdMA multiplied into the ratio : and therefore the horary

variation of the inclination will be to 33" 10'" 33 iv . as the area HpMA
TZ Pz?

multiplied into AZ X ^ X to AT 3 .

Cor. 2. And, therefore, if the earth and nodes were after every hour
drawn back from their new and instantly restored to their old places, so as
their situation might continue given for a whole periodic month together,
the whole variation of the inclination during that month would be to 33'“

442

THE MATHEMATICAL PRINCIPLES

[Book III

10'" 33 iv . as the aggregate of all the areas HpM//, generated in the time ot
one revolution of the point p (with due regard in summing to their proper

p p

signs + -*■), multiplied into AZxTZx to Mjo x AT 3 ; that is, a a

P/3

the whole circle QA qa multiplied into AZ X TZ X to Mp X AT 3 ,

P/3

that is, as the circumference QA^a multiplied into AZ X TZ X ^ to
2Mp X AT 2 .

Cor. 3. And, therefore, in a given position of the nodes, the mean ho¬
rary variation, from which, if uniformly continued through the whole
month, that menstrual variation might be generated, is to 33" 10"' 33 iv . as

P /3 AZ x TZ

AZ X TZ X ~ to 2AT 2 , or as Pp X — 7 ^ to PG X 4AT; that

PG

rAT

is (because Pp is to PG as the sine of the aforesaid inclination to the ra¬

dius, and

AZ X TZ

~Tat”

to 4AT as the sine of double the angle ATh to four

times the radius), as the sine of the same inclination multiplied into the
sine of double the distance of the nodes from the sun to four times the
square of the radius. •

Cor. 4. Seeing the horary variation of the inclination, when the nodes
are in the quadratures, is (by this Prop.) to the angle 33" 1.0 " 33 iv . as IT
Pp\. ..IT X TG w Pp

X AZ X TG X —^ to AT 3 , that is, as

lu

4 AT

X to 2AT, that

ru

is, as the sine of double the distance of the moon from the quadratures
multiplied into to twice the radius, the sum of all the horary varia¬
tions during the time that the moon, in this situation of the nodes, passes
from the quadrature to the syzygy (that is, in the space of 1 77} hours) will
be to the sum of as many angles 33" 10"' 33 lv . or 5878", as the sum of all
the sines of double the distance of the moon from the quadratures multi-
Pp

plied into to the sum of as many diameters; that is, as the diameter
1 PG

Pp

multiplied into ~ to the circumference; that is, if the inclination be 5°

l',,as 7 X rfllo to 22, or as 278 to 10000. And, therefore, the whole
variation, composed out of the sum of all the horary variations in the
aforesaid time, is 103", or 2' 43".

Book HI.]

OF NATURAL PHILOSOPHY.

443

PROPOSITION XXXV. PROBLEM XVI.

To a given time to find the inclination of the moon's orbit to the plant

of the ecliptic.

Let AD be the sine of the greatest inclination, and AB the sine of the
least. Bisect BD in C; and round the centre C, with the interval BC,
describe the circle BGD. In AC take CE in the same proportion to EB

as EB to twice BA. And if to the time given we set off the angle AEG
equal to double the distance of the nodes from the quadratures, and upon
AD let fall the perpendicular GH, AH will be the sine of the inclination
required.

For GE 2 is equal to GH 2 + HE 2 = BHD + HE 2 = HBD + HE 2
— BH 2 = HBD + BE 2 — 2BH X BE = BE 2 + 2EC X BH == 2EC
X AB + 2EC X BH = 2EC X AH; wherefore since 2EC is given. GE 2
will be as AH. Now let AE«* represent double the distance of the nodes
from the quadratures, in a given moment of time after, and the arc G^, on
account of the given angle GE»*, will be as the distance GE. But Wi is
to Go* as GH to GO, and, therefore, H h is as the rectangle GH x G«i or

GH GH

GH X GE, that is, as == X GE 2 , or x AH; that is, as AH and

txrj (jrJjj

the sine of the angle AEG conjunctly. If, therefore, in any one case, AH
be the sine of inclination, it will increase by the same increments as the
sine of inclination doth, by Cor. 3 of the preceding Prop, and therefore will
always continue equal to that sine. But when the point G falls upon
either point B or D, AH is equal to this sine, and therefore remains always
equal thereto. Q..E.D.

In this demonstration I have supposed that the angle BEG, representing
double the distance of the nodes from the quadratures, increaseth uniform¬
ly ; for I cannot descend to every minute circumstance of inequality. Now
suppose that BEG is a right angle, and that Gg is in this case the ho¬
rary increment of double the distance of the nodes from the sun; then, by
Cor. 3 of the last Prop, the horary variation of the inclination in the same
case will be to 33" 10"' 33 iv . as the rectangle of AH, the sine of the incli¬
nation, into the sine of the right ano-le BEG, double the distance of the
nodes from the sun, to four times the square of the radius; that is, as AH,

444

THE MATHEMATICAL PRINCIPLES

[Book )1L

the sine of the mean inclination, to four times the radius; that is, seeing
the mean inclination is about 5° S£, as its sine 896 to 40000, the quad¬
ruple of the radius, or as 224 to 10000. But the whole variation corres¬
ponding to BD, the difference of the sines, is to this horary variation as
the diameter BD to the arc G^, that is, conjunctly as the diameter BD to
the semi* circumference BGD, and as the time of 2079 T 7 „ hours, in which
the node proceeds from the quadratures to the syzysries, to one hour, that
is as 7 to 11, and 2079 T 7 ¥ to 1. Wherefore, compounding all these pro¬
portions, we shall have the whole variation BD to 33" 10'" 33 iv . as 224 X
7 X 2079 T \ to 110000, that is, as 29645 to 1000; and from thence that
variation BD will come out 16' 23i".

And this is the greatest variation of the inclination, abstracting from
the situation of the moon in its orbit: for if the nodes are in the syzygies,
the inclination suffers no change from the various positions of the moon.
But if the nodes are in the quadratures, the inclination is less when the
moon is in the syzygies than when it is in the quadratures by a difference
of 2' 43", as we shewed in Cor. 4 of the preceding Prop.; and the whole
mean variation BD, diminished by l 7 211", the half of this excess, becomes
15' 2", when the moon is in the quadratures; and increased by the same,
becomes 17' 45" when the moon is in the syzygies. If, therefore, the
moon be in the syzygies, the whole variation in the passage of the nodes
from the quadratures to the syzygies will be 17' 45"; and, therefore, if the
inclination be 5° 17' 20", when the nodes are in the syzygies, it will be 4°
59' 35" when the nodes are in the quadratures and the moon in the syzy¬
gies. The truth of all which is confirmed by observations.

Now if the inclination of the orbit should be required when the moon is
in the syzygies, and the nodes any where between them and the quadratures,
let AB be to AD as the sine of 4° 59' 35" to the sine of 5° 17' 20", and
take the angle AEG equal to double the distance of the nodes from the
quadratures; and AH will be the sine of the inclination desired. To this
inclination of the orbit the inclination of the same is equal, when the moon
is 90° distant from the nodes. In other situations of the moon, this men¬
strual inequality, to which the variation of the inclination is obnoxious in
the calculus of the moon’s latitude, is balanced, and in a manner took off,
by the menstrual inequality of the motion of the nodes (as we said
before), and therefore may be neglected in the computation of the said
latitude.

SCHOLIUM.

By these computations of the lunar motions I was willing to shew that
by the theory of gravity the motions of the moon could be calculated from
their physical causes. By the same theory I moreover found that the an¬
nual equation of the mean motion of the moon arises from the various

Book III.]

OF NATURAL PHILOSOPHY

445

dilatation which the orbit of the moon suffers from the action of the sun
according to Cor. 6, Prop. LXVI. Book 1. The force of this action is
greater in the perigeon sun, and dilates the moon’s orbit; in the apogeon
sun it is less, and permits the orbit to be again contracted. The moon
moves slower in the dilated and faster in the contracted orbit; and the
annual equation, by w T hich this inequality is regulated, vanishes in the
apogee and perigee of the sun. In the mean distance of the sun from the
earth it arises to about 11' 50" ; in other distances of the sun it is pro¬
portional to the equation of the sun’s centre, and is added to the mean
motion of the moon, w r hile the earth is passing from its aphelion to its
perihelion, and subducted wdiile the earth is in the opposite semi-circle.
Taking for the radius of the orbis rnagnus 1000, and 16J for the earth’s
eccentricity, this equation, when of the greatest magnitude, by the theory
of gravity comes out IP 49". But the eccentricity of the earth seems to
be something greater, and with the eccentricity this equation will be aug¬
mented in the same proportion. Suppose the eccentricity 16}^, and the
greatest equation will be IP 51".

Farther; I found that the apogee and nodes of the moon move fastei
in the perihelion of the earth, where the force of the sun’s action is greater,
than in the aphelion thereof, and that in the reciprocal triplicate propor¬
tion of the earth’s distance from the sun ; and hence arise annual equa¬
tions of those motions proportional to the equation of the sun’s centre.
Now the motion of the sun is in the reciprocal duplicate proportion of the
earth’s distance from the sun ; and the- greatest equation of the centre
which this inequality generates is 1° 56’ 20", corresponding to the above-
mentioned eccentricity of the sun, 16}-|. But if the motion of the sun
had been in the reciprocal triplicate proportion of the distance, this ine¬
quality would have generated the greatest equation 2° 54' 30" ; and there¬
fore the greatest equations which the inequalities of the motions of the
moon’s apogee and nodes do generate are to 2° 54' 30" as the mean diur¬
nal motion of the moon’s apogee and the mean diurnal motion of its
nodes are to the mean diurnal motion of the sun. Whence the greatest
equation of the mean motion of the apogee comes out 19' 43", and the
greatest equation of the mean motion of the nodes 9' 24". The former
equation is added, and the latter subducted, while the earth is passing
from its perihelion to its aphelion, and contrariwise when the earth is in
the opposite semi-circle.

By the theory of gravity I likewise found that the action of the sun
upon the moon is something greater when the transverse diameter of the
moon’s orbit passeth through the sun than when the same is perpendicu¬
lar upon the line which joins the earth and the sun; and therefore the
moon’s orbit is something larger in the former than in the latter case.
And hence arises another equation of the moon’s mean motion, depending

446

THE MATHEMATICAL PRINCIPLES

[Book III

upon the situation of the moon’s apogee in respect of the sun, which is in
its greatest quantity when the moon’s apogee is in the octants of the sun,
and vanishes when the apogee arrives at the quadratures or syzygies; and
it is added to the mean motion while the moon’s apogee is passing from
the quadrature of the sun to the syzygy, and subducted while the apogee
is passing from the syzygy to the quadrature. This equation, which I
shall call the semi-annual, when greatest in the octants of the apogee,
arises to about 3' 45", so far as I could collect from the phenomena: and
this is its quantity in the mean distance of the sun from the earth. But
it is increased and diminished in the reciprocal triplicate proportion of
the sun’s distance, and therefore is nearly 3' 34" when that distance is
greatest, and 3' 56" when least. But when the moon’s apogee is without
the octants, it becomes less, and is to its greatest quantity as the sine of
double the distance of the moon’s apogee from the nearest syzygy or quad¬
rature to the radius.

By the same theory of gravity, the action of the sun upon the moon is
something greater when the line of the moon’s nodes passes through the
sun than when it is at right angles with the line which joins the sun and
the earth ; and hence arises another equation of the moon’s mean motion,
which I shall call the second semi-annual; and this is greatest when the
nodes are in the octants of the sun, and vanishes when they are in the
syzygies or quadratures; and in other positions of the nodes is propor¬
tional to the sine of double the distance of either node from the nearest
syzygy or quadrature. And it is added to the mean motion of the moon,
if the sun is in antecedentia , to the node which is nearest to him, and
subducted if in consequential and in the octants, where it is of the
greatest magnitude, it arises to 47" in the mean distance of the sun from
the earth, as I find from the theory of gravity. In other distances of the
sun, this equation, greatest in the octants of the nodes, is reciprocally as
the cube of the sun’s distance from the earth ; and therefore in the sun’s
perigee it comes to about 49", and in its apogee to about 45".

By the same theory of gravity, the moon’s apogee goes forward at the
greatest rate when it is either in conjunction with or in opposition to the
sun, but in its quadratures with the sun it goes backward; and the ec¬
centricity comes, in the former case, to its greatest quantity ; in the latter
to its least, by Cor. 7, 8, and 9, Prop. LXYI, Book 1. And those ine¬
qualities, by the Corollaries we have named, are very great, and generate
the principal which I call the semi-annual equation of the apogee; and
this semi-annual equation in its greatest quantity comes to about 12° 18',
as nearly as I could collect from the phenomena. Our countryman,
Horrox , was the first who advanced the theory of the moon’s moving in
an ellipsis about the earth placed in its lower focus. Dr. Halley improved
the notion, by putting the centre of the ellipsis in an epicycle whose cen-

OF NATURAL PHILOSOPHY.

447

Book III.]

tre is uniformly revolved about the earth; and from the motion in this
epicycle the mentioned inequalities in the progress and regress of the apo¬
gee, and in the quantity of eccentricity, do arise. Suppose the mean dis¬
tance of the moon from the earth to be divided into 100000 parts, and
let T represent the earth, and TC the moon’s mean eccentricity of 5505
such parts. Produce TO to B, so as CB may be the sine of the greatest
semi-annual equation 12° IS' to the radius TC; and the circle BDA de¬
scribed about the centre 0, with the
interval CB, will be the epicycle
spoken of, in which the centre of the
moon’s orbit is placed, and revolved
according to the order of the letters
BDA. Set off the angle BCD equal
to twice the annual argument, or
twice the distance of the sun’s true place from the place of the moon’s
apogee once equated, and CTD will be the semi-annual equation of the
moon’s apogee, and TD the eccentricity of its orbit, tending to the place
of the apogee now twice equated. But, having the moon’s mean motion,
the place of its apogee, and its eccentricity, as well as the longer axis of
its orbit 200000, from these data the true place of the moon in its orbit,
together with its distance from the earth, may be determined by the
methods commonly known.

In the perihelion of the earth, where the force of the sun is greatest,
the centre of the moon’s orbit moves faster about the centre C than in the

aphelion, and that in the reciprocal triplicate proportion of the sun's dis¬
tance from the earth. But, because the equation of the sun’s centre is
included in the annual argument, the centre of the moon’s orbit moves
faster in its epicycle BDA, in the reciprocal duplicate proportion of the
sun’s distance from the earth. Therefore, that it may move yet faster in
the reciprocal simple proportion of the distance, suppose that from D, the
centre of the orbit, a right line DE is drawn, tending towards the moon’s
apogee once equated, that is, parallel to TO; and set off the angle EDE
equal to the excess of the aforesaid annual argument above the distance
of the moon’s apogee from the sun’s perigee in consequentia ; or, which
comes to the same thing, take the angle CDF equal to the complement of
the sun’s true anomaly to 360°; and let DF be to DC as twice the eccen¬
tricity of the orbis magnus to the sun’s mean distance from the earth,
and the sun’s mean diurnal nut ion from the moon’s apogee to the sun’s
mean diurnal motion from its own apogee conjunctly, that is, as 33| to
1000, and 52' 27" 16"' to 50' 8" 10'"-conjunctly, or as 3 to 100; and
imagine the centre of the moon’s orbit placed in the point F to be revolved
in an epicycle whose centre is D, and radius DF, while the point D moves
in the circumference of the circle DABD : for by this means the centre of

44S

THE MATHEMATICAL PRINCIPLES

[Book III

the moon’s orbit comes to describe a certain curve line about the centre C
with a velocity which will be almost reciprocally as the cube of the sun’s
distance from the earth, as it ought to be.

The calculus of this motion is difficult, but may be rendered more easy
by the following approximation. Assuming, as above, the moon’s mean
distance from the earth of 100000 parts, and the eccentricity TC of 5505
Buch parts, thedine CB or CD will be found 1172f, and DF 35i of those
parts: and this line DF at the distance TC subtends the angle at the earth,
which the removal of the centre of the orbit from the place D to the place
F generates in the motion of this centre; and double this line DF in a
parallel position, at the distance of the upper focus of the moon’s orbit from
the earth, subtends at the earth the same angle as DF did before, which
that removal generates in the motion of this upper focus; but at the dis¬
tance of the moon from the earth this double line 2DF at the upper focus,
in a parallel position to the first line DF, subtends an angle at the moon,
which the said removal generates in the motion of the moon, which angle
may be therefore called the second equation of the moon’s centre; and this
equation, in the mean distance of the moon from the earth, is nearly as the
sine of the angle wdiich that line DF contains with the line drawn from
the point F to the moon, and when in its greatest quantity amounts to 2'
25". But the angle which the line DF contains with the line drawn from
the point F to the moon is found either by subtracting the angle EDF
from the mean anomaly of the moon, or by adding the distance of the moon
from the sun to the distance of the moon’s apogee from the apogee of the
sun; and as the radius to the sine of the angle thus found, so is 2' 25" to
the second equation of the centre: to be added, if the forementioned sum
be less than a semi-circle; to be subducted, if greater. And from the moon’s
place in its orbit thus corrected, its longitude may be found in the syzygies
of the luminaries.

The atmosphere of the earth to the height of 35 or 40 miles refracts the
sun’s light. This refraction scatters and spreads the light over the earth’s
shadow; and the dissipated*light near the limits of the shadow dilates the
shadow. Upon which account, to the diameter of the shadow, as it comes
out by the parallax, I add 1 or li minute in lunar eclipses.

But the theory of the moon ought to be examined and proved from the
phenomena, first in the syzygies, then in the quadratures, and last of all
in the octants; and whoever pleases to undertake the work will find it
not amiss to assume the following mean motions of the sun and moon at
the Royal Observatory of Greenwich , to the last day of December at noon,
anno 1700, O.S. viz. The mean motion of the sun V5> 20° 43' 40", and of
its apogee 25 7° 44' 30"; the mean motion of the moon ox 15° 21' 00";
of its apogee, X 8° 20' 00"; and of its ascending node ft 27° 24' 20";
and the difference of meridians betwixt the Observatory at Greenwich and

OF NATURAL PHILOSOPHY.

449

Book III.]

the Royal Observatory at Paris , 0 h . 9' 20"': but the mean motion >f the
moon and of its apogee are not yet obtained with sufficient accuracy.

PROPOSITION XXXYI. PROBLEM XVII.

To find the force of the sun to move the sea.

The sun’s force ML or PT to disturb the motions of the moon, was (by
Prop. XXV.) in the moon’s quadratures, to the force of gravity with us, as
1 to 638092,6; and the force TM — LM or 2PK in the moon’s syzygies
is double that quantity. But, descending to the surface of the earth, these
forces are diminished in proportion of the distances from the centre of the
earth, that is, in the proportion of 60| to 1 ; and therefore the former force
on the earth’s surface is to the force of gravity as 1 to 38604600; and by
this force the sea is depressed in such places as are 90 degrees distant from
the sun. But by the other force, which is twice as great, the sea is raised
not only in the places directly under the sun, but in those also which are
directly opposed to it; and the sum of these forces is to the force of gravity
as 1 to 12868200. And because the same force excites the same motion,
whether it depresses the waters in those places which are 90 degrees distant
from the sun, or raises them in the places which are directly under and di¬
rectly opposed to the sun, the aforesaid sum will be the total force of the
sun to disturb the sea, and will have the same effect as if the whole was
employed in raising the sea in the places directly under and directly op¬
posed to the sun, and did not act at all in the places which are 90 degrees
removed from the sun.

And this is the force of the sun to disturb the sea in any given place,
where the sun is at the same time both vertical, and in its mean distance
from the earth. In other positions of the sun, its force to raise the sea is
as the versed sine of double its altitude above the horizon of the place di¬
rectly, and the cube of the distance from the earth reciprocally.

Cor. Since the centrifugal force of the parts of the earth, arising from
the earth’s diurnal motion, which is to the force of gravity as 1 to 289,
raises the waters under the equator to a height exceeding that under the
poles by S5472 Paris feet, as above, in Prop. XIX., the force of the sun,
which we have now shewed to be to the force of gravity as 1 to 12S6S200,
and therefore is to that centrifugal force as 289 to 12S6S200, or as 1 to
44527, will be able to raise the waters in the plaoes directly under and di¬
rectly opposed to the sun to a height exceeding that in the places which are
90 degrees removed from the sun only by one Paris foot and 113 ^ inchss;
for this measure is to the measure of 85472 feet as 1 to 44527.

PROPOSITION XXXVII. PROBLEM XVIII.

To find the force of the moon to move the sea.

The force of the moon to move the sea is to be deduced from its proper*

29

450

THE MATHEMATICAL PRINCIPLES

[Book III

tion to the force of the sun, and this proportion is to be collected from the
proportion of the motions of the sea, which are the effects of those forces.
Before the mouth of the river Avon, three miles below Bristol , the height
of the ascent of the water in the vernal and autumnal syzygies of the lu¬
minaries (by the observations of Samuel Sturmy ) amounts to about 45
feet, but in the quadratures to 25 only. The former of those heights ari¬
ses from the sum of the aforesaid forces, the latter from their difference.
If, therefore, S and L are supposed to represent respectively the forces of
the sun and moon while they are in the equator, as well as in their mean
distances from the earth, we shall have L + S to L— S as 45 to 25, or as
9 to 5.

At Plymouth (by the observations of Samuel Colepress ) the tide in its
mean height rises to about 16 feet, and in the spring and autumn the
height thereof in the syzygies may exceed that in the quadratures by more
than 7 or 8 feet. Suppose the greatest difference of those heights to be 9
feet, and L -4- S will be to L — S as 20| to ll|, or as 41 to 23; a pro¬
portion that agrees well enough with the former. But because of the great
tide at Bristol , we are rather to depend upon the observations of Sturmy ;
and, therefore, till we procure something that is more certain, we shall use
the proportion of 9 to 5.

But because of the reciprocal motions of the waters, the greatest tides do
not happen at the times of the syzygies of the luminaries, but, as we have
said before, are the third in order after the syzygies; or (reckoning from
the syzygies) follow next after the third appulse of the moon to the me¬
ridian of the place after the syzygies; or rather (as Sturmy observes) are
the third after the day of the new or full moon, or rather nearly after the
twelfth hour from the new or full moon, and therefore fall nearly upon the
forty-third hour after the new or full of the moon. But in this port they
fall out about the seventh hour after the appulse of the moon to the me¬
ridian of the place; and therefore follow next after the appulse of the
moon to the meridian, when the moon is distant from the sun, or from op¬
position with the sun by about IS or 19 degrees in consequeutia. So the
summer and winter seasons come not to their height in the solstices them¬
selves, but when the sun is advanced beyond the solstices by about a tenth
part of its whole course, that is, by about 36 or 37 degrees. In like man¬
ner, the greatest tide is raised after the appulse of the moon to the meridian
of the place, when the moon has passed by the sun, or the opposition thereof .
by .about a tenth part of the whole motion from one greatest tide to the
next follorving greatest tide. Suppose that distance about 1S^ degrees;
and the sun’s force in this distance of the moon from the syzygies and
quadratures will be of less moment to augment and diminish that part ot
the motion of the sea which proceeds from the motion of the moon than in
the syzygies and quadratures themselves in the proportion of the radius tu

OF NATURAL PHILOSOPHY

451

Book III.]

the co-sine of double this distance, or of an angle of 3/ degrees; that is, in
proportion of 10000000 to 79S 5355; and, therefore, in the preceding an¬
alogy, in place of S we must put 0,?9S6355S.

But farther; the force of tfle moon in the quadratures must be dimin¬
ished, on account of its declination from the equator; for the moon in
those quadratures, or rather in lSi degrees past the quadratures, declines
from the equator by about 23° 13'; and the force of either luminary to
move the sea is diminished as it declines from the equator nearly in the
duplicate proportion of the co-sine of the declination ; and therefore the
force of the moon in those quadratures is only 0.S570327J,; whence we
have L+0,7986355S to 0,S570327L — 0,7986355S as 9 to 5.

Farther yet; the diameters of the orbit in which the moon should move,
setting aside the consideration of eccentricity, are one to the other as 69
to 70; and therefore the moon’s distance from the earth in the svzygies
is to its distance in the quadratures, cccteris paribus , as 69 to 70 ; and its
distances, when 181 degrees advanced beyond the syzygies, where the great¬
est tide was excited, and when 18^ degrees passed by the quadratures,
where the least tide was produced, are to its mean distance as 69,09S747
and 69,897345 to 69i. But the force of the moon to move the sea is in
the reciprocal triplicate proportion of its distance; and therefore its
forces, in the greatest and least of those distances, are to its force in its
mean distance as 0.9S30427 and 1,017522 to I. From whence we have
1,0175221, X 0,79S6355S. to 0,9S30427 X 0,8570327L — 0,7986355S
as 9 to 5; and S to J, as 1 to 4,4815. Wherefore since the force of the
sun is to the force of gravity as 1 to 12868200, the moon’s force will be
to the force of gravity as 1 to 2871400.

Cor. 1. Since the waters excited by the sun’s force rise to the height of
a foot and 11 ¥ ’ ¥ inches, the moon’s force will raise the same to the height
of 8 feet and 7 -^- inches; and the joint forces of botli will raise the same
to the height of lOi feet; and when the moon is in its perigee to the
height of 12i feet, and more, especially when the wind sets the same way
as the tide. And a force of that quantity is abundantly sufficient to ex¬
cite all the motions of the sea, and agrees well with the proportion of ;
those motions; for in such seas as lie free and open from east to west, as-
in the Pacific sea, and in those tracts of the Atlantic and Ethiopic seas
which lie without the tropics, the waters commonly rise to 6, 9,-.12, cr-15 :
feet; but in the Pacific sea, which is of a greater depth, as well as-of a
larger extent, the tides are said to be greater than in the Atlantic and /
Ethiopic seas; for to have a full tide raised, an extent of sea from east'to
west is required of no less than 90 degrees. In the Ethiopic sea, the watera-
rise to a less height within the tropics than in the temperate zones, be¬
cause of the narrowness of the sea between Africa and the southern parts
of America . In the middle of the open sea the waters cannot rise with-

152

THE MATHEMATICAL PRINCIPLES

[Book 111.

out falling together, and at the same time, upon both the eastern and west¬
ern shores, when, notwithstanding, in our narrow seas, they ought to fall
on those shores by alternate turns ; upon Avhich account there is com¬
monly but a small flood and ebb in such islands as lie far distant from
the continent. On the contrary, in some ports, where to fill and empty
the bays alternately the waters are with great violence forced in and out
through shallow channels, the flood and ebb must be greater than ordinary ;
as at Plymouth and Chepstow Bridge in England , at the mountains of
St. Michael , and the town of Aura?iches, in Normandy , and at Cambaia
and Pegu in the East Indies. In these places the sea is hurried in and
<^ut with such violence, as sometimes to lay the shores under water, some¬
times to leave them dry for many miles. Nor is this force of the influx
and efflux to be broke till it has raised and depressed the waters to 30, 40,
or 50 feet and above. And a like account is to be given of long and shal¬
low channels or straits, such as the Magellanic straits, and those chan¬
nels which environ England. The tide in such ports and straits, by the
violence of the influx and efflux, is augmented above measure. But on
such shores as lie towards the deep and open sea with a steep descent,
where the waters may freely rise and fall without that precipitation of
influx and efflux, the proportion of the tides agrees with the forces of the
sun and moon.

Cor. 2. Since the moon’s force to move the sea is to the force of gravity
as 1 to 2871400, it is evident that this force is far less than to appear
sensibly in statical or hydrostatical experiments, or even in those of pen¬
dulums. It is in the tides only that this force shews itself by any sensi¬
ble effect.

Cor. 3. Because the force of the moon to move the sea is to the like
ff>rce of the sun as 4,4815 to 1, and those forces (by Cor. 14, Prop. LXYI,
Book 1) are as the densities of the bodies of the sun and moon and the
cubes of their apparent diameters conjunctly, the density of the moon will
be to the density of the sun as 4,4815 to 1 directly, ^nd the cube of the
moon’s diameter to the cube of the sun’s diameter inversely ; that is (see¬
ing the mean apparent diameters of the moon and sun are 3P 161", and
32' 12"), as 4891 to 1000. But the density of the sun was to the den¬
sity of the earth as 1000 to 4000; and therefore the density of the moon
is to the density of the earth as 4891 to 4000, or as 11 to 9. Therefore
the body of the moon is more dense and more earthly than the earth
itself.

Cor. 4. And since the true diameter of the moon (from the observations
of astronomers) is to the true diameter of the earth as 100 to 365, the
mass of matter in the moon will be to the mass of matter in the earth as
l to 39,788.

Cor. 5. And the accelerative gravity on the surface of the moon will be

OF NATURAL PHILOSOPHY.

453

3ook III.]

about three times less than the accelerative gravity on the surface of thr
earth.

Cor. 6. And the distance of the moons centre from the centre of the
earth will be to the distance of the moon’s centre from the common centre
of gravity of the earth and moon as 40,788 to 39,788.

Cor. 7. And the mean distance of the centre of the moon from the
centre of the earth will be (in the moon’s octants) nearly 60f of the great¬
est semi-diameters of the earth; for the greatest semi-diameter of the
earth was 1965S600 Paris feet, and the mean distance of the centres of
the earth and moon, consisting of 60f such semi-diameters, is equal to
1187379440 feet. And this distance (by the preceding Cor.) is to the dis¬
tance of the moon’s centre from the common centre of gravity of the
earth and moon as 40.788 to 39,7SS ; which latter distance, therefore, is
1158268534 feet. And since the moon, in respect of the fixed stars, per¬
forms its revolution in 27 d . 7 h . 43f ', the versed sine of that angle which
the moon in a minute of time describes is 12752341 to the radius
1000,000000,000000; and as the radius is to this versed sine, so are
115S268534 feet to 14,7706353 feet. The moon, therefore, falling tow¬
ards the earth by that force which retains it in its orbit, would in one
minute of time describe 14,7706353 feet; and if we augment this force
in the proportion of 17S|| to 177§§, we shall have the total force of
gravity at the orbit of the moon, by Cor. Prop. Ill; and the moon falling
by this force, in one minute of time would describe 14,8538067 feet. And
at the 60th part of the distance of the moon from the earth's centre, that
is, at the distance of 197S96573 feet from the centre of the earth, a body
falling by its weight, would, in one second of time, likewise describe
14,S53S067 feet. And, therefore, at the distance of 19615800, which
compose one mean semi-diameter of the earth, a heavy body would de¬
scribe in falling 15,11175, or 15 feet, 1 inch, and 4-^ lines, in the same
time. This will be the descent of bodies in the latitude of 45 degrees.
And by the foregoing table, to be found under Prop. XX, the descent in
the latitude of Paris will be a little greater by an excess of about f parts
of a line. Therefore, by this computation, heavy bodies in the latitude of
Paris falling in vacuo will describe 15 Paris feet, 1 inch, 4|f lines, very
nearly, in one second of time. And if the gravity be diminished by tak¬
ing away a quantity equal to the centrifugal force arising in that latitude
«rom the earth’s diurnal motion, heavy bodies falling there will describe
in one second of time 15 feet, 1 inch, and l£ line. And with this velo¬
city heavy bodies do really fall in the latitude of Paris , as we have shewn
above in Prop. IV and XIX.

Cor. 8. The mean distance of the centres of the earth and moon in the
syzygies of the moon is equal to 60 of the greatest semi-diameters of the
earth, subducting only about one 30th par 1 ; of a semi-diameter: and in the

454

THE MATHEMATICAL PRINCIPLES

[Book III.

moon’s quadratures the mean distance of the same centres is 60| such semi¬
diameters of the earth; for these two distances are to the mean distance oi
the moon in the octants as 69 and 70 to 69|, by Prop. XXVIII.

Cor. 9. The mean distance of the centres of the earth and moon in the
syzygies of the moon is 60 mean semi-diameters of the earth, and a 10th
part of one semi-diameter; and in the moon’s quadratures the mean dis¬
tance of the same centres is 61 mean semi-diameters of the earth, subduct¬
ing one 30th part of one semi-diameter.

Cor. 10. In the moon’s syzygies its mean horizontal parallax in the lat¬
itudes of 0, 30, 38, 45, 52, 60, 90 degrees is 57' 20", 57' 16", 57' 14", 57
12", 57' 10", 57' 8", 57' 4", respectively.

In these computations I do not consider the magnetic attraction of the
earth, whose quantity is very small and unknown: if this quantity should
ever be found out, and the measures of degrees upon the meridian, the
lengths of isochronous pendulums in different parallels, the laws of the mo¬
tions of the sea, and the moon’s parallax, with the apparent diameters of
the sun and moon, should be more exactly determined from phenomena: we
should then be enabled to bring this calculation to a greater accuracy.

PROPOSITION XXXVIII. PROBLEM XIX.

To find the figure of the moon's body.

If the moon’s body were fluid like our sea, the force of the earth to raise
that fluid in the nearest and remotest parts would be to the force of the
moon by which our sea is raised in the places under and opposite to the
moon as the accelerative gravity of the moon towards the earth to the ac¬
celerative gravity of the earth towards the moon, and the diameter of the
moon to the diameter of. the earth conjunctly; that is, as 39,7S8 to 1, and
LOO to 365 conjunctly, or as 1081 to 100. Wherefore, since our sea, by
the force of the moon, is raised to Sf feet, the lunar fluid would be raised
by the force of the earth to 93 feet; and upon this account the figure of
the moon would be a spheroid, whose greatest diameter produced would
pass through the centre of the earth, and exceed the diameters perpendicu¬
lar thereto by 186 feet. Such a figure, therefore, the. moon affects, and
must have put on from the beginning. Q.E.I.

Cor. Hence it is that the same face of the moon always respects the
earth; nor can the body of the moon possibly rest in any other position,
but would return always by a libratory motion to this situation; but those
librations, however, must be exceedingly slow, because of the weakness of
the forces which excite them; so that the face of the moon, which should
be always obverted to the earth, may, for the reason assigned in Prop. XVIL
be turned towards the other focus of the moon’s orbit, without being im¬
mediately drawn hack, and converted again towards the earth.

Book III.]

OF NATURAL PHILOSOPHY.

45n

LEMMA L

If APEp represent the earth uniformly dense, marked with the centre C,
the poles P, p, and the equator AE; and if about the centre C, with
the radius CP, ice suppose the sphere Pape to be described, and Q R to
denote the plane on which a right line, drawn from the centre of the
sun to the centre of the earth, insists at right angles; and further
suppose that the several particles of the ichole exterior earth PapAPepE,
without the height of the said sphere, endeavour to recede towards this
side and that side from the plane QR, every particle by a force pro¬
portional to its distance from that plane ; I say, in the first place, that
the whole force and efficacy of all the particles that are situate in AE,
the circle of the equator, and disposed uniformly without the globe,
encompassing the same after the manner of a ring, to icheel the earth
about its centre, is to the whole force and efficacy of as many particles
in that point A of the equator which is at the greatest distance from
the plane QJR,, to wheel the earth about its centre icith a like circular
motion, as l to 2. And that circular motion will be performed about
an axis lying in the common section of the equator and the plane CAR.

For let there be described from the centre K, with the diameter IL, the
semi-circle INL. Suppose the semi-circumference INL to be divided
into innumerable equal parts, and from the several parts N to the diameter

IL let fall the sines NM. Then the sums of the squares of all the sines
NM will be equal to the sums of the squares of the sines KM, and both
sums together will be equal to the sums of the squares of as many semi¬
diameters KN; and therefore the sum of the squares of all the sines NM
will be but half so great as the sum of the squares of as many semi-diam¬
eters KN.

Suppose now the circumference of the circle AE to be divided into the
like number of little equal parts, and from every such part F a perpen¬
dicular FG to be let fall upon the plane QR, as well as the perpendicular
AH from the point A. Then the force by which the particle F recedes

456 THE MATHEMATICAL PRINCIPLES [BOOK IIL

from the plane QR will (by supposition) be as that perpendicular FG; and
this force multiplied by the distance CG will represent the power of the
particle F to turn the earth round its centre. And, therefore, the power
of a particle in the place F will be to the power of a particle in the place
A as FG X GC to AH X HC; that is, as FC 2 to AC 2 : and therefore
the whole power of all the particles F, in their proper places F, will be to
the power of the like number of particles in the place A as the sum of all
the FC 2 to the sum of all the AC 2 , that is (by what we have demonstrated
before), as 1 to 2. Q.E.D.

And because the action of those particles is exerted in the direction of
lines perpendicularly receding from the plane QR, and that equally from
each side of this plane, they will wheel about the circumference of the circle
of the equator, together with the adherent body of the earth, round an axis
which lies as well in the plane QR as in that of the equator.

LEMMA II.

The same things still supposed, I say, in the second place , that the total
force or power of all the particles situated every ichere about the sphere
to turn the earth about the said axis is to the whole force of the like
number of particles, uniformly disposed round the whole circumference
of the equator AE in the fashion of a ring, to turn the whole earth
about with the like circular motion, as 2 to 5.

For let IK be any lesser circle parallel to
the equator AE, and let LZ be any two equal
particles in this circle, situated without the
sphere Vape] and if upon the plane QR,
which is at right angles with a radius drawn
to the sun, we let fall the perpendiculars LM,
Im , the total forces by which these particles
recede from the plane QR will be propor¬
tional to the perpendiculars LM, Im. Let
the right line L l be drawn parallel to the
plane P ape, and bisect the same in X; and
through the point X draw N n parallel to the plane QR, and meeting the
perpendiculars LM, Im, in N and n • and upon the plane QR let fall the
perpendicular XY. And the contrary forces of the particles L and l to
wheel about the earth contrariwise are as LM X MC, and Im X mC ; that
is, as LN X MC + NM X MC, and In X mC — nm X mC ; or LN X
MC + NM X MC, and LN X mC — NM X mC, and LN X M m —
NM X MC + raC, the difference of the two, is the force of both taken
together to turn the earth round. The affirmative part of this difference
LN x Mm, or 2LN X NX, is to 2AH X HC, the force of two particles
of the same size situated in A., as LX 2 to AC* ; and the negative part NM

Book II1.J

OB NATURAL PHILOSOPHY.

457

X MC + 7/iCj or 2XY X CY, is to 2AH X HC, the force of the same
two particles situated in A, as CX 2 to AC 2 . And therefore the difference
of the parts, that is, the force of the two particles L and /, taken together,
to wheel the earth about, is to the force of two particles, equal to the
former and situated in the place A, to turn in like manner the earth round,
as LX 2 — CX 2 to AC 2 . But if the circumference IK of the circle IK
is supposed to be divided into an infinite number of little equal parts L,
all the LX 2 will be to the like number of IX 2 as 1 to 2 (by Lem. 1); and
to the same number of AC 2 as IX 2 to 2AC 2 ; and the same number oi
CX 2 to as many AC 2 as 2CX 2 to 2AC 2 . Wherefore the united forces
of all the particles in the circumference of the circle IK are to the joint
forces of as many particles in the place A as IX 2 — 2CX 2 to 2AC 2 ; and
therefore (by Lem. 1) to the united forces of as many particles in the cir¬
cumference of the circle AE as IX 2 — 2CX 2 to AC 2 .

Now if P p, the diameter of the sphere, is conceived to be divided into
an infinite number of equal parts, upon which a like number of circles
IK are supposed to insist, the matter in the circumference of every circle
IK will be as IX 2 ; and therefore the force of that matter to turn the
earth about will be as IX 2 into IX 2 — 2CX 2 : and the force of the same
matter, if it was situated in the circumference of the circle AE, would be as
IX 2 into AC 2 . And therefore the force of all the particles of the whole
matter situated without the sphere in the circumferences of all the circle?
is to the force of the like number of particles situated in the circumfer¬
ence of the greatest circle AE as all the IX 2 into IX 2 —2CX 2 to as
many IX 2 into AC 2 ; that is, as all the AC 2 — CX 2 into AC 2 — 3CX 2
to as many AC 2 — CX 2 into AC 2 ; that is, as all the AC 4 —4AC 2 X
CX 2 + 3CX 4 to as many AC 4 —AC 2 X CX 2 ; that is, as the whole
fluent quantity, whose fluxion is AC 4 —4AC 2 X CX 2 + 3CX 4 , to the
whole fluent quantity, whose fluxion is AC 4 — AC 2 X CX 2 ; and, there¬
fore, by the method of fluxions, as AC 4 X CX— fAC 2 X CX 3 +
|CX S to AC 4 X CX— £AC 2 X CX 3 ; that is, if for CX we write the
whole Op, or AC, as T 4 T AC 5 to |AC 5 ; that is, as 2 to 5. Q,.E.D.

LEMMA III.

The same things still supposed , I say, in the third place, that the mo¬
tion of the whole earth about the axis above-named arising from the
motions of all the particles , will be to the motion of the aforesaid ring
about the same axis in a proportion compounded of the proportion of
the matter in the earth to the matter in the ring ; and the proportion,
of three squares of the quadrantal arc of any circle to two squares
of its diameter, that is, in the proportion of the matter to the matter,
and of the number 925275 to the number 1000000.

For the motion of a cylinder revolved about its quiescent axis is to the

458

THE MATHEMATICAL PRINCIPLES

[Book III.

motion of the inscribed sphere revolved together with it as any four equal
squares to three circles inscribed in three of those squares; and the mo¬
tion of this cylinder is to the motion of an exceedingly thin ring sur-
rounding both sphere and cylinder in their common contact as double the
matter in the cylinder to triple the matter in the rir^ ; and this motion
of the ring, uniformly continued about the axis of the cylinder, is to the
uniform motion of the same about its own diameter performed in the
same periodic time as the circumference of a circle to double its diameter.

HYPOTHESIS II.

If the other parts o f the earth were taken away, and the remaining ring
was carried alone about the sun in the orbit of the earth by the annual
motion , while by the diurnal motion it ivas in the mean time revolved
about its own axis inclined to the plane of the ecliptic by an angle
of 23|- degrees , the motion of the equinoctial points icould be the
same , whether the ring were fluid , or whether it consisted of a hard
and rigid matter.

PROPOSITION XXXIX. PROBLEM XX.

To find the precession of the equ inoxes.

The middle horary motion of the moon’s nodes in a circular orbit, when
the nodes are in the quadratures, was 16" 35'" 16 iv . 36 v .; the half of
which, 8" 17"' 3S V . 1S V . (for the reasons above explained) is the mean ho¬
rary motion of the nodes in such an orbit, which motion in a whole side¬
real year becomes 20° 11' 46". Because, therefore, the nodes of the moon
in such an orbit would be yearly transferred 20? 11' 46" in antecedentia ;
and, if there were more moons, the motion of the nodes of every one (by
Cor. 16, Prop. LXVI, Book 1) would be as its periodic time; if upon the
surface of the earth a moon was revolved in the time of a sidereal day,
the annual motion of the nodes of this moon would be to 20° 11' 46" as
23 h . 56', the sidereal day, to 27\ 7 h . 43', the periodic time of our moon,
that is, as 1436 to 39343. And the same thing would happen to the
nodes of a ring of moons encompassing the earth, whether these moons
did not mutually touch each the other, or whether they were molten, and
formed into a continued ring, or whether that ring should become rigid
and inflexible.

Let us, then, suppose that this ring is in quantity of matter equal to
the whole exterior earth P<//?APe/?E, which lies without the sphere P ape
(see fig. Lem. II); and because this sphere is to that exterior earth as «C-
to AC 2 — aC 2 , that is (seeing PC or aC the least semi-diameter of the
earth is to AC the greatest semi-diameter of the same as 229 to 230), as
52441 to 459 ; if this ring encompassed the earth round the equator, and
both together were revolved about the diameter of the ring, the motion of

OF NATURAL PHILOSOPHY.

459

Book III.]

the ring (by Lem. Ill) would be to the motion of the inner sphere as 459
to 52441 and 1000000 to 925275 conjunctly, that is, as 4590 to 485223;
and therefore the motion of the ring would be to the sum of the motions
of both ring and sphere as 4590 to 489813. Wherefore if the ring ad¬
heres to the sphere, and communicates its motion to the sphere, by which
its nodes or equinoctial points recede, the motion remaining in the ring will
be to its former motion as 4590 to 489S13; upon which account the
motion of the equinoctial points will be diminished in the same propor¬
tion. Wherefore the annual motion of the equinoctial points of the body,
composed of both ring and sphere, will be to the motion 20° 11' 46" as
1436 to 39343 and 4590 to 489S13 conjunctly, that is, as 100 to 292369.
But the forces by which the nodes of a number of moons (as we explained
above), and therefore by which the equinoctial points of the ring recede
(that is, the forces 3IT, in fig. Prop. XXX), are in the several particles
as the distances of those particles from the plane Q,R ; and by these forces
the particles recede from that plane: and therefore (by Lem. II) if the
matter of the ring was spread all over the surface of the sphere, after the
fashion of the figure Pd/joAPejoE, in order to make up that exterior part
of the earth, the total force or power of all the particles to wheel about
the earth round any diameter of the equator, and therefore to move the
equinoctial points, would become less than before in the proportion of 2 to
5. Wherefore the annual regress of the equinoxes now would be to 20°
IP 46" as 10 to 73092; that is. would be 9" 56'" 50 iv .

But because the plane of the equator is inclined to that of the ecliptic,
this motion is to be diminished in the proportion of the sine 91706
(which is the co-sine of 23} deg.) to the radius 100000; and the remain¬
ing motion will now be 9" 7'" 20 iv . which is the annual precession of the
equinoxes arising from the force of the sun.

But the force of the moon to move the sea was to the force of the sun
nearly as 4,4815 to 1; and the force of the moon to move the equinoxes
is to that of the sun in the same proportion. Whence the annual precession
of the equinoxes proceeding from the force of the moon comes out 40"
52"' 52 iv . and the total annual precession arising from the united forces
of both will be 50" 00'" 12 iv . the quantity of which motion agrees with
the phenomena; for the precession of the equinoxes, by astronomical ob¬
servations, is about 50" yearly.

If the height of the earth at the equator exceeds its height at the
poles by more than 17} miles, the matter thereof will be more rare near
the surface than at the centre; and the precession of the equinoxes will
be augmented by the excess of height, and diminished by the greater rarity.

And now we have described the system of the sun, the earth, moon,
and planets, it remains that we add something about the comets.

460

THE MATHEMATICAL PRINCIPLES

[Book IIL

LEMMA IV

That the comets are higher than the moon , and in the regiotis of the

planets,

As the comets were placed by astronomers above the moon, because they
were found to have no diurnal parallax, so their annual parallax is a con¬
vincing proof of their descending into the regions of the planets; for all
the comets which move in a direct course according to the order of the
signs, about the end of their appearance become more than ordinarily slow
or retrograde, if the earth is between them and the sun; and more than
ordinarily swift, if the earth is approaching to a heliocentric opposition
with them; whereas, on the other hand, those which move against the or¬
der of the signs, towards the end of their appearance appear swifter than
they ought to be, if the earth is between them and the sun; and slower,
and perhaps retrograde, if the earth is in the other side of its orbit. And
these appearances proceed chiefly from the diverse situations which the
earth acquires in the course of its motion, after the same manner as it hap¬
pens to the planets, which appear sometimes retrograde, sometimes more
slowly, and sometimes more swiftly, progressive, according as the motion of
the earth falls in with that of the planet, or is directed the contrary way.
If the earth move the same way with the comet, but, by an angular motion
about the sun, so much swifter that right lines drawn from the earth to
the comet converge towards the parts beyond the comet, the comet seen
from the earth, because of its slower motion, will appear retrograde; and
even if the earth is slower than the comet, the motion of the earth being
subducted, the motion of the comet will at least appear retarded; but if the
earth tends the contrary way to that of the comet, the motion of the comet
will from thence appear accelerated; and from this apparent acceleration,
or retardation, or regressive motion, the distance of the comet may be in-
g F _c_B_A. ferred in this manner. Let TQA,

N \. 1 7 7/ TQB, TQ,C, be three observed lon-

\ 1 / / gitudes of the comet about the time

\\ \ / °f a PP ear ing, an d TQ,F its

\\ \ / y krst observed longitude before its

\\ 1 / / disappearing. Draw the right line

1//^^ ABC, whose parts AB, BC, inter-

<2 cepted between the right lines QA

and Q.B, Q,B and Q.C, may be one to the other as the two times between
the three first observations. Produce AC to G, so as AG may be to A B
as the time between the first and last observation to the time between the
first and second; and join QG. Now if the comet did move uniformly in
a right line, and the earth either stood still, or was likewise carried for-
wards in a right line by an uniform motion, the angle TQG would be the

Book 111.]

OF NATURAL PHILOSOPHY.

401

longitude of the comet at the time of the last observation. The angle,
therefore, FQ,G, which is the difference of the longitude, proceeds from the
inequality of the motions of the comet and the earth; and this angle, if
the earth and comet move contrary ways, is added to the angle TQ,G, and
accelerates the apparent motion of the comet; but if the comet move the
same way with the earth, it is subtracted, and either retards the motion oi
the comet, or perhaps renders it retrograde, as we have but now explained.
This angle, therefore, proceeding chiefly from the motion of the earth, is
justly to be esteemed the parallax of the comet; neglecting, to wit, some
little increment or decrement that may arise from the unequal motion of
the comet in its orbit; and from this parallax we thus deduce the distance
of the comet. Let S represent the sun, acT v
the or bis magnus, a the earth’s place in the
first observation, c the place of the earth in \\
the third observation, T the place of the \\
earth in the last observation, and TT a right

line drawn to the beginning of Aries. Set \ N. TT

off the angle TTV equal to the angle TQ,F,

that is, equal to the longitude of the comet --

at the time when the earth is in T; join ac, y
and produce it to g, so as ag may be to ac / Y

as AG to AG ; and g will be the place at I s 1)

which the earth would have arrived in the \ j a

time of the last observation, if it had con- \ y

tinued to move uniformly in the right line -^

ac. Wherefore, if we draw gT parallel to TT, and make the angle T^Y
equal to the angle TQ,G, this angle TgV will be equal to the longitude of
the comet seen from the place g, and the angle TYg* will be the parallax
which arises from the earth’s being transferred from the place g into the
place T; and therefore V will be the place of the comet in the plane of the
ecliptic. And this place Y is commonly lower than the orb of Jupiter.

The same thing may be deduced from the incurvation of the way of the
comets; for these bodies move almost in great circles, while their velocity
is great; but about the end of their course, when that part of their appa¬
rent motion w r hich arises from the parallax bears a greater proportion to
their whole apparent motion, they commonly deviate from those circles, and
when the earth goes to one side, they deviate to the other: and this deflex¬
ion, because of its corresponding with the motion of the earth, must arise
chiefly from the parallax; and the quantity thereof is so considerable, as,
by my computation, to place the disappearing comets a good deal lower
than Jupiter. Whence it follows that when they approach nearer to us in
their perigees and perihelions they often descend below the orbs of Mars
and the inferior planets.

462

THE MATHEMATICAL PRINCIPLES

[Book JII

The near approach of the comets is farther confirmed from the light of
their heads; for the light of a celestial body, illuminated by the sun, and
receding to remote parts, is diminished in the quadruplicate proportion of
the distance; to wit, in one duplicate proportion, on account of the increase
of the distance from the sun, and in another duplicate proportion, on ac¬
count of the decrease of the apparent diameter. Wherefore if both the
quantity of light and the apparent diameter of a comet are given, its dis¬
tance will be also given, by taking the distance of the comet to the distance
of a planet in the direct proportion of their diameters and the reciprocal
subduplicate proportion of their lights. Thus, in the comet of the year
1682, Mr. Flam steel observed with a telescope of 16 feet, and measured
with a micrometer, the least diameter of its head, 2' 00; but the nucleus
or star in the middle of the head scarcely amounted to the tenth part of
this measure; and therefore its diameter was only 11" or 12"; but in the
light and splendor of its head it surpassed that of the comet in the year
16S0, and might be compared with the stars of the first or second magni¬
tude. Let us suppose that Saturn with its ring was about four times more
lucid; and because the light of the ring was almost equal to the light of
the globe within, and the apparent diameter of the globe is about 21", and
therefore the united light of both globe and ring would be equal to the
light of a globe whose diameter is 3U", it follows that the distance of th
comet was to the distance of Saturn as 1 to ^4 inversely, and 12" to 30
directly; that is, as 24 to 30, or 4 to 5. Again; the comet in the month
of April 1665, as Hevelius informs us, excelled almost all the fixed stars
in splendor, and even Saturn itself, p.s being of a much more vivid colour;
for this comet w^as more lucid than that other which had appeared about
the end of the preceding year, and had been compared to the stars of the
first magnitude. The diameter of its head was about 6'; but the nucleus,
compared with the planets by means of a telescope, was plainly less than
Jupiter; and sometimes judged less, sometimes judged equal, to the globe
of Saturn within the ring. Since, then, the diameters of the heads of the
comets seldom exceed 8' or 12', and the diameter of the nucleus or central
star is but about a tenth or perhaps fifteenth part of the diameter of the
head, it appears that these stars are generally of about the same apparent
magnitude with the planets. But in regard that their light may be often
compared with the light of Saturn, yea, and sometimes exceeds it, it is evi¬
dent that all comets in their perihelions must either be placed below or not
far above Saturn; and they are much mistaken who remove them almost
as far as the fixed stars; for if it was so, the comets could receive no more
light from our sun than our planets do from the fixed stars.

So far we have gone, without considering the obscuration which comets
suffer from that plenty of thick smoke which encompasseth their heads,
and through which the heads always shew dull, as through i cloud; for by

BoOIC 111.] Or NATURAL PHILOSOPHY. 463

how much the more a body is obscured by this smoke, by so much the more
near it must be allowed to come to the sun, that it may vie with the plan¬
ets in the quantity of light which it reflects. Whence it is probable that
the comets descend far below the orb of Saturn, as we proved before fron
their parallax. But, above all, the thing is evinced from their tails, which
must be owing either to the sun’s light reflected by a smoke arising from
them, and dispersing itself through the tether, or to the light of their own
heads. In the former case, we must shorten the distance of the comets,
lest we be obliged to allow that the smoke arising from their heads is
propagated through such a vast extent of space, and with such a velocity
and expansion as will seem altogether incredible; in the latter case, the
whole light of both head and tail is to be ascribed to the central nucleus.
But, then, if we suppose all this light to be united and condensed within
the disk of the nucleus, certainly the nucleus will by far exceed Jupiter
itself in splendor, especially when it emits a very large and lucid tail If,
therefore, under a less apparent diameter, it reflects more light, it must be
much more illuminated by the sun, and therefore much nearer to it; and
the same argument will bring down the heads of comets sometimes within
the orb of Venus, viz., when, being hid under the sun’s rays, they emit such
huge and splendid tails, like beams of fire, as sometimes they do; for if all
that light was supposed to be gathered together into one star, it would
sometimes exceed not one Venus only, but a great many such united
into one.

Lastly; the same thing is inferred from the light of the heads, which
increases in the recess of the comets from the earth towards the sun, and
decreases in their return from the sun towards the earth ; for so the comet
of the year 1665 (by the observations of Hevelius), from the time that it
was first seen, was always losing of its apparent motion, and therefore had
already passed its perigee; but yet the splendor of its head was daily in¬
creasing, till, being hid under the sun’s rays, the comet ceased to appear.
The comet of the year 16S3 (by the observations of the same Hevelius),
about the end of July, when it first appeared, moved at a very slow rate,
advancing only about 40 or 45 minutes in its orb in a day’s time; but
from that time its diurnal motion was continually upon the increase, till
Septe?nber 4, when it arose to about 5 degrees; and therefore, in all this
interval of time, the comet was approaching to the earth. Which is like¬
wise proved from the diameter of its head, measured with a micrometer;
for, August 6, Hevelius found it only 6' 05", including the coma, which,
September 2 he observed to be 9' 07 ", and therefore its head appeared far
less about the beginning than towards the end of the motion; though
about the beginning, because nearer to the sun it appeared far more lucid
than towards the end, as the same Hevelius declares. Wherefore in all
this interval of time, on account of its recess from the sun, it decreased

464

THE MATHEMATICAL PRINCIPLES

[Book III.

in splendor, notwithstanding its access towards the earth. The comet of
the year 1618, about the middle of December , and that of the year 1680,
about the end of the same month, did both move with their greatest velo¬
city, and were therefore then in their perigees ; but the greatest splendor
of their heads was seen two weeks before, when they had just got clear of
the sun’s rays ; and the greatest splendor of their tails a little more early,
when yet nearer to the sun. The head of the former comet (according to
the observations of Cysatns ), December 1, appeared greater than the stars
of^tlie first magnitude: and, December 16 (then in the perigee), it was
but little diminished in magnitude, but in the splendor and brightness of
its light a great deal. January 7, Kepler , being uncertain about the
head, left off observing. December 12, the head of the latter comet was
seen and observed by Mr. Flamsted, when but 9 degrees distant from the
sun; which is scarcely to be done in a star of the third magnitude. De¬
cember 15 and 17, it appeared as a star of the third magnitude, its lustre
being diminished by the brightness of the clouds near the setting sun.
December 26, when it moved with the greatest velocity, being almost in
its perigee, it was less than the mouth of Pegasus , a star of the third
magnitude. January 3. it appeared as a star of the fourth. January 9,
as one of the fifth. January 13, it was hid by the splendor of the moon,
then in her increase. January 25, it was scarcely equal to the stars of
the seventh magnitude. If we compare equal intervals of time on one
side and on the other from the perigee, we shall find that the head of the
comet, which at both intervals of time was far, but yet equally, removed
from the earth, and should have therefore shone with equal splendor, ap¬
peared brightest on the side of the peri-gee towards the sun, and disap¬
peared on the other. Therefore, from the great difference of light in the
one situation and in the other, we conclude the great vicinity of the sun
and comet in the former; for the light of comets uses to be regular, and
to appear greatest -when the heads move fastest, and are therefore in their
perigees; excepting in so far as it is increased by their nearness to the
sun. -

Cor. 1. Therefore the comets shine by the sun’s light, which they reflect.

Cor. 2. Prom what has been said, we may likewise understand why
comets are so frequently seen in that hemisphere in which the sun is, and
so seldom in the other. If they were visible in the regions far above
Saturn, they would appear more frequently in the parts opposite to the
sun; for such as were in those parts would be nearer to the earth, whereas
the presence of the sun must obscure and hide those that appear in the
hemisphere in which he is. Yet, looking over the history of comets, I
find that four or five times more have been seen in the hemisphere towards
the sun than in the opposite hemisphere; besides, without doubt, not a
few, which have been hid by the light of the sun: for comets descending

Book III.] o* natural philosophy. 465

into our parts neither emit tails, nor are so well illuminated by the sun,
as to discover themselves to our naked eyes, until they are come nearer to
us than Jupiter. But the far greater part of that spherical space, which
is described about the sun with so small an interval, lies on that side of
the earth which regards the sun .; and the comets in that greater part are
commonly more strongly illuminated, as being for the most part nearer to
the sun.

Cor. 3. Hence also it is evident that the celestial spaces are void of
resistance; for though the comets are carried in oblique paths, and some¬
times contrary to the course of the planets, yet they move every way with
the greatest freedom, and preserve their motions for an exceeding long
time, even where contrary to the course of the planets. I am out in my
judgment if they are not a sort of planets revolving in orbits returning
into themselves with a perpetual motion j for, as to what some writers
contend, that they are no other than meteors, led into this opinion by the
perpetual changes that happen to their heads, it seems to have no founda¬
tion ; for the heads of comets are encompassed with huge atmospheres,
and the lowermost parts of these atmospheres must be the densest; and
therefore it is in the clouds only, not in the bodies of the comets them¬
selves, that these changes are seen. Thus the earth, if it was viewed from
the planets, would, without all doubt, shine by the light of its clouds, and
the solid body would scarcely appear through the surrounding clouds.
Thus also the belts of Jupiter are formed in the clouds of thari planet,
for they change their position one to another, and the solid body of Jupiter
is hardly to be seen through them ; and much more must the bodies of
comets be hid under their atmospheres, which are both deeper and thicker.

PROPOSITION XL. THEOREM XX.

That the comets move in some of the conic sections , having their foci
in, the centre of the sun; and by radii drawn to the sun describe
a^eas proportional to the times.

This proposition appears from Cor. 1, Prop. XIII, Book 1, compared
with Prop. VIII, XII, and XIII, Book m.

Cor. 1. Hence if comets are revolved in orbits returning into them¬
selves, those orbits will be ellipses; and their periodic times be to the
periodic times of the planets in the sesquiplicate proportion of their prin¬
cipal axes. And therefore the comets, which for the most part of their
course are higher than the planets, and upon that account describe orbits
with greater axes, will require a longer time to finish their revolutions.
Thus if the axis of a comet’s orbit was four times greater than the axis
of the orbit of Saturn, the time of the revolution of the comet would be
to the time of the revolution of Saturn, that is, to 30 years, as 4 4

(or 8) to 1, and would therefore be 240 vears.

30

466

THE MATHEMATICAL PRINCIPLES [BOOK III.

Cor. 2. But their orbits will be so near to parabolas, that parabolas
may be used for them without sensible error.

Cor. 3. And, therefore, by Cor. 7, Prop. XVI, Book 1, the velocity of
every comet will always be to the velocity of any planet, supposed to be
revolved at the same distance in a circle about the sun, nearly in the sub¬
duplicate proportion of double the distance of the planet from the centre
of the sun to the distance of the comet from the sun’s centre, very nearly.
Let us suppose the radius of the orbis mag mis , or the greatest semi-
diameter of the ellipsis which the earth describes, to consist of 100000000
parts; and then the earth by its mean diurnal motion will describe
1720212 of those parts, and 716751 by its horary motion. And there¬
fore the comet, at the same mean distance of the earth from the sun, with
a velocity which is to the velocity of the earth as ^ 2 to 1, would by its
diurnal motion describe 2432747 parts, and 1013641 parts by its horary
motion. But at greater or less distances both the diurnal and horary
motion will be to this diurnal and horary motion in the reciprocal subdu¬
plicate proportion of the distances, and is therefore given.

Cor. 4. Wherefore if the lains rectum of the parabola is quadruple of
the radius of the orbis magnns, and the square of that radius is sup¬
posed to consist of 100000000 parts, the area which the comet will daily
describe by a radius drawn to the sun will be 12163731 parts, and the
horary area will be 506S21 parts. But, if the latns rectum is greater
or less in any proportion, the diurnal and horary area will be less or
greater in the subduplicate of the same proportion reciprocally.

LEMMA V.

To find a curve line of the parabolic kind which shall pass through any
given number of points.

Let those points be A, B, C, D, E, P, &c., and from the same to any
right line HN, given in position, let fall as many perpendiculars AH, BI,
OK, DL, EM, FN, &c.

b 2b 3b 4b 5b

c 2 c 3 c 4 c
d 2d 3d

f

Case 1. If HI, IK, KL, &c., the intervals of the points H, I, K, L, M
N, &c., are equal, take b, 2b, 3b, 4b, 5b, &c., the first differences of the per¬
pendiculars AH, BI, CK, &c.; their second differences c, 2c, 3c, 4c, &c.;
their third, d, 2d, 3d , &c., that is to say, so as AH — BI may be == b, BI

Book III.]

OF NATURAL PHILOSOPHY.

4 67

— CK = 2b, CK — DL = 3b, DL + EM = 4 b, — EM + FN = 5b,
&c.; then b — 2b = c, &c., and so on to the last difference, which is here
/*. Then, erecting any perpendicular RS, which may be considered as an
ordinate of the curve required, in order to find the length of this ordinate,
suppose the intervals HI. IK, KL, LM, (fee., to be units, and let AH = a,

— HS = p, \p into — IS = q, \q into + SK = r, fr into + SL = s,
\s into -f SM = t; proceeding, to wit, to ME, the last perpendicular but
one, and prefixing negative signs before the terms HS, IS, (fee., which lie
from S towards A; and affirmative signs before the terms SK, SL, (fee.,
which lie on the other side of the point S; and, observing well the signs,
RS will be = a + bp + cq + dr + es 4- ft, + (fee.

Case 2. But if HI, IK, (fee., the intervals of the points H, I, K, L, (fee.,
are unequal, take b, 2b, 3b, 4b, 5b, (fee., the first differences of the perpen¬
diculars AH, BI, CK, (fee., divided by the intervals between those perpen¬
diculars ; c, 2c, 3c, 4c, (fee., their second differences, divided by the intervals
between every two; d, 2d, 3d, (fee., their third differences, divided by the
intervals between every three; e, 2e, (fee., their fourth differences, divided
by the intervals between every four; and so forth ; that is, in such manner,

BI — CK

CK — DL

, AH —BI ^
that b may be =-HX —’

b — 2b 0 26 — 36 0 3b — 4b , 7 c — 2c

c > 2c ■ j 3c = tFtvt > (fee., then d 777 * 2d

jg -—j 3b =-gc -1 .' then

HK
2c — 3c

IM

IL

KM

HL

(fee. And those differences being found, let AH be = a, —

HS = p,p into — IS — q, q into + SK = r, r into + SL =. s, s into
-b SM = t ; proceeding, to wit, to ME, the last perpendicular but one:,
and the ordinate RS will be = a + bp + cq + dr + es + //, + (fee.

Cor. Hence the areas of all curves may be nearly found; for if some •
number of points of the curve to be squared are found, and a parabola be
supposed to be drawn through those points, the area of this parabola will!
be nearly the same with the area of the curvilinear figure proposed to be
squared : but the parabola can be always squared geometrically by methods
vulgarly known.

LEMMA VI.

Certain observed places of a comet being given, to find the place of the
same to any intermediate given time.

Let HI, IK, KL, LM (in the preceding Fig.), represent the times between
the observations ; HA, IB, KC, LD, ME, five observed longitudes of the
comet; and HS the given time between the first observation and the longi¬
tude required. Then if a regular curve ABCDE is supposed to be drawn
through the points A, B, C, D, E, and the ordinate RS is found out by the
preceding lemma, RS will be the longitude required.

4fiS THE MATHEMATICAL PRINCIPLES [BOOK III.

After the same method, from five observed latitudes, we may find the
latitude to a given time.

If the differences of the observed longitudes are small, suppose of 4 or 5
degrees, three or four observations will be sufficient to find a new longitude
and latitude; but if the differences are greater, as of 10 or 20 degrees, five
observations ought to be used.

LEMMA VII.

Through a given point P to draw a right line BC, tohose parts PB, PC,
cut off by two right lines AB, AC, given in position , may be one to the
other in. a given proportion.

From the given point P suppose any right line
PD to be drawn to either of the right lines given,
as AB; and produce the same towards AC, the
other given right line, as far as E, so as PE may
be to PD in the given proportion. Let EC be
parallel to AD. Draw CPB, and PC will be to PB
as PE to PD. Q.E.F.

LEMMA VIII.

Let ABC be a parabola , having its focus in S. By the chord AC bi¬
sected in I cut of the segment ABCI, tohose diameter is Ip and vertex
/'. In Ip produced take pO equal to one half of Ip. Join OS, and
produce it to so as S£ may be equal to 2SO. Now, supposing a comet
to revolve in. the arc CBA, draw £B, cutting AC in E; I say , the point
E will cut off from the chord AC the segment AE, nearly proportional
to the time.

For if we join EO, cutting the parabolic arc ABC in Y, and draw p~K
touching the same arc in the vertex p, and meeting EO in X, the curvi¬
linear area AEXjuA will be to the curvilinear area ACYpA as AE to AC ;
and, therefore, since the triangle ASE is to the triangle ASC in the same
proportion, the whole area ASEX/jA will be to the whole area ASC Y/aA as

o

OF NATURAL PHILOSOPHY.

469

Book II Lj

AE to AC. But, because £0 is to SO as 3 to 1, and EO to XC in the same
proportion, SX will be parallel to EB ; and, therefore, joining BX, the tri¬
angle SEB will be equal to the triangle XEB. Wherefore if to the area
ASEX.^A we add the triangle EXB, and from the sum subduct the triangle
SEB, there will remain the area ASBXuA, equal to the area ASEX/iA. and
therefore in proportion to the area ASCY//A as AE to AC. But the area
ASBYuA is nearly equal to the area ASBXiuA; and this area ASBY//A
is to the area ASCY.uA as the time of description of the arc AB to the
time of description of the whole arc AC; and, therefore, AE is to AC
nearly in the proportion of the times. Q.E.D.

Cor. When the point B falls upon the vertex j a of the parabola, AE is
to AC accurately in the proportion of the times.

SCHOLIUM.

If we join ju| cutting AC in d 7 and in it take £n in proportion to juB as
27MI to 16Mfq and draw B?/, this Bn will cut the chord AC, in the pro¬
portion of the times, more accurately than before; but the point n is to be
taken beyond or on this side the point £, according as the point B is
more or less distant from the principal vertex of the parabola than the
point y.

LEMMA IX.

AI :

The right lines I// and yM, and the length are equal among them-

selves.

For 4.S y is the latus rectum of the parabola belonging to the vertex y.

LEMMA X.

Produce Sy to N and P, so as yN may be one third of yl, and SP may
be to SN as SN to Sy ; and in the time that a comet would describe
the arc AyC. if it was supposed to move always forwards icith the ve¬
locity which it hath in a height equal to SP, it would describe a length
equal to the chord AC.

For if the comet with the velocity
which it hath in y was in the said time
supposed to move uniformly forward in
the right line which touches the parabola
in jw, the area which it would describe by
a radius drawn to the point S would be
equal to the parabolic area ASCyA ; and
therefore the space contained under the
length described in the tangent and the
length Si u would be to the space contained under the lengths AC and SM as the

4/0

THE MATHEMATICAL PRINCIPLES

[Book 111

area ASC yA to the triangle A SC, that is, as SN to SM. Wherefore AC
is to the length described in the tangent as Sj u to SN. But since the ve¬
locity of the comet in the height SP (by Cor. 6, Prop. XVI., Book I) is to
the velocity of the same in the height Sin the reciprocal subduplicate
proportion of SP to Sy, that is, in the proportion of Sy to SN, the length
described with this velocity will be to the length in the same time described
in the tangent as S y to SN. Wherefore since AC, and the length described
with this new velocity, are in the same proportion to the length described
in the tangent, they must be equal betwixt themselves. Q.E.D.

Cor. Therefore a comet, with that velocity which it hath in the height
Sy + ffy, would in the same time describe the chord AC nearly.

LEMMA XI.

If a comet void of all motion was let fall from the height SN, or Sy -f
}Jy, towards the sun , and was still impelled to the sun by the same
force uniformly continued by which it was impelled at first , the same ,
in one half of that time in. which it might describe the arc AC in its
own orbit , woidd, in descending describe a space equal to the length
Iy.

For in the same time that the comet would require to describe the para¬
bolic arc AC, it would (by the last Lemma), with that velocity which it
hath in the height SP, describe the chord AC ; and, therefore, (by Cor. 7,
Prop. XVI, Book 1), if it was in the same time supposed to revolve by the
force of its own gravity in a circle whose semi-diameter was SP, it would
describe an arc of that circle, the length of which would be to the chord
of the parabolic arc AC in the subduplicate proportion of 1 to 2. Where¬
fore if with that weight, which in the height SP it hath towards the sun,
it should fall- from that height towards the sun, it would (by Cor. 9,
Prop. XVI, Book 1) in half the said time describe a space equal to the
square of half the said chord applied to quadruple the height SP, that is,

AP

it would describe the space But since the weight of the comet

towards the sun in the height SN is to
the weight of the same towards the
sun in the height SP as SP to Sy, the
comet, by the weight which it hath in
the height SN. in falling from that
height towards the sun, would in the

AP

same time describe the space that

is, a space equal to the length ly ot
n M. Q.E.D

Book III.]

OF NATURAL PHILOSOPHY.

47.

PROPOSITION XLI. PROBLEM XXI.

From three observations given to determine the orbit of a comet moving

in a parabola.

This being a Problem of very great difficulty, I tried many methods of
resolving it; and several of these Problems, the composition whereof I
have given in the first Book, tended to this purpose. But afterwards I
contrived the following solution, which is something more simple.

Select three observations distant one from another by intervals of time
nearly equal; but let that interval of time in which the comet moves
more slowly be somewhat greater than the other; so, to wit, that the dif¬
ference of the times may be to the sum of the times as the sum of the

n

times to about 600 days; or that the point E may fall upon M nearly,
and may err therefrom rather towards I than towards A. If such direct
observations are not at hand, a new place of the comet must be found, by
Lem. VI.

Let S represent the sun; T, t, r, three places of the earth in the orbis
magnus; TA, /B, tC, three observed longitudes of the comet; Y the
time between the first observation and the second; W the time between
the second and the third; X the length which in the whole time V + W

the comet might describe with that velocity which it hath in the mean
distance of the earth from the sun, which length is to be found by Cor. 3,

4 72

THE MATHEMATICAL PRINCIPLES

[Book ILL

Prop. XL, Book III; and tY a perpendicular upon the chord Tr. In the
mean observed longitude /B take at pleasure the point B, for the place of
the comet in the plane of the ecliptic; and from thence, towards the sun
S, draw the line BE, which may be to the perpendicular tY as the content
under SB and S/ 2 to the cube of the hypothenuse of the right angled tri¬
angle, whose sides are SB, and the tangent of the latitude of the comet in
the second observation to the radius /B. And through the point E (by
Lemma VII) draw the right line AEC, whose parts xYE and EC, terminat¬
ing in the right lines TA and tC. may be one to the other as the times V
and YV : then A and C will be nearly the places of the comet in the plane
of the ecliptic in the first and third observations, if B was its place
rightly assumed in the second.

Upon AC, bisected in I, erect the perpendicular Ii. Through B draw
the obscure line Bi parallel to AC. Join the obscure line Si, cutting AC
in A, and complete the parallelogram ii A/z. Take la equal to 3IA ; and
through the sun S draw the obscure line'o£ equal to 3Sa + 3 iA. Then,
cancelling the letters A, E, C, I, from the point B towards the point £,
draw the new obscure line BE, which may be to the former BE in the
duplicate proportion of the distance BS to the quantity S/z + -i iA. And
through the point E draw again the right line AEC by the same rule as
before; that is, so as its parts AE and EC may be one to the other as the
times V and W between the observations. Thus A and C will be the
places of the comet more accurately.

Upon AC, bisected in I, erect the perpendiculars AM, CN, 10, of which
AM and CN may be the tangents of the latitudes in the first and third ob¬
servations, to the radii TA and tC. Join MN, cutting 10 in O. Draw the
rectangular parallelogram zIA^, as before. In IA produced take ID equal to
S [i + f iA. Then in MN, towards N, take MP, which may be to the
above found length X in the subduplicate proportion of the mean distance
of the earth from the sun (or of the semi-diameter of the orbis wagnus)
to the distance OD. If the point P fall upon the point N; A, B, and C,
will be three places of the comet, through which its orbit is to be described
In the plane of the ecliptic. But if the point P falls not upon the point
N, in the right line AC take CG equal to NP, so as the points G and V
may lie on the same side of the line NC.

By the same method as the points E, A, C, G, were found from the as¬
sumed point B, from other points b and j3 assumed at pleasure, find out the
new points e, a , c, g ; and £, a, k } y. Then through G, g, and y, draw the
circumference of a circle Ggy, cutting the right line rC in Z: and Z will
be one place of the comet in the plane of the ecliptic. And in AC, ac, cm,
making AF, af, aty, equal respectively to CG, eg , /cy; through the points F,
f, and 0, draw the circumference of a circle F/V>, cutting the right line AT
tn X; and the point X will be another place of the comet in the plane of

OF NATURAL PHILOSOPHY.

473

Book IIL]

the ecliptic. And at the points X and Z, erecting the tangents of the
latitudes of the comet to the radii TX and tZ, two places of the comet in
its own orbit will be determined. Lastly, if (by Prop. XIX., Book 1) to
the focus S a parabola is described passing through those two places, this
parabola will be the orbit of the comet. Q.E.L

The demonstration of this construction follows from the preceding Lem¬
mas, because the right line AC is cut in E in the proportion of the times,
by Lem. VIL, as it ought to be, by Lem. VIII.; and BE, by Lem. XI., is a
portion of the right line BS or B£ in the plane of the ecliptic, intercepted
between the arc ABC and the chord AEC; and MP (by Cor. Lem. X.) is
the length of the chord of that arc, which the comet should describe in its
proper orbit between the firs; and third observation, and therefore is equal
to MN, providing B is a true place of the comet in the plane of the
ecliptic.

But it will be convenient to assume the points B, b , /3, not at random,
but nearly true. If the angle AQ/, at which the projection of the orbit in
the plane of the ecliptic cuts the right line tB, is rudely known, at that
angle with Bt draw the obscure line AC, which may be to f T r in the sub¬
duplicate proportion of SQ, to St ; and, drawing the right line SEB so as
its part EB may be equal to the length Yt, the point B will be determined,
which we are to use for the first time. Then, cancelling the right line
AC, and drawing anew AC according to the preceding construction, and,
moreover, finding the length MP, in /B take the point b, by this rule, that,
if TA and tC intersect each other in Y, the distance Y b may be to the
distance YB in a proportion compounded of the proportion of MP to MN
and the subduplicate proportion of SB to S b. And by the same method
you may find the third point 1 3, if you please to repeat the operation the
third time; but if this method is followed, two operations generally will be
sufficient; for if the distance B b happens to be very small, after the points
Fand G, g, are found, draw the right lines Ff and Gg*, and they will
cut TA and rC in the points required, X and Z.

EXAMPLE.

Let the comet of the year 1680 be proposed. The following table shews
the motion thereof, as observed by Flamsted , and calculated afterwards by
him from his observations, and corrected by Dr. Halley from the same ob¬
servations.

THE MATHEMATICAL PRINCIPLES

rBooK m.

471

Time.

Sun’s

Longitude.

| Comet’s

Appar.

True.

Longitude.

Lai. N.

h. »

h. ' "

o / »

o / H

O t tt

1680, Dec. 12

4.46

4.46. 0

V? 1.51.23

V? 6.32.30

8.28. 0

21

6.32£

6.36.59

11.06 44

~ 5.08.12

21.42.13

24

6.12

6.17.52

14.09.26

18.49.23

25.23. 5

26

5.14

5 20.44

16.09.22

28.24.13

27.00 52

29

7.55

8.03.02

19.19.43

X 13.10.41

28.09.58

30

8.02

S. 10.26

20.21.09

17.38.20

28.11.53

1681. Jan. 5

5.51

6.01.38

26.22.18

°P 8.48.53

26.15. 7

9

6.49

7 00.53

~ 0.29.02

18.44.04

24.11.56

10

5.54

6.06.10

1.27.43

20.40.50

23.43.52

13

6.56

7.08.55

4.33.20

25.59.48

22.17.28

25

7.44

7.58.42

16.45.36

» 9.35. 0

17.56.30

30

8.07

8.21.53

21.49.58

13.19.51

16.42 18

Feb. 2

6.20

6.34.51

24.46.59

15.13 53

16.04.’ 1

5

6.50

7.04.41

! 27 49.51

16.59 06

15.27. 3

To these you may add some observations of mine.

Ap.

Time

Comet’s

Longitude.

55 ■

Lat. N.

h. '

o 9 n

o t /;

1681, Feb. 25

8.30

« 26.18.35

12.46.46

27

8.15

27.04.30

12.36.12

Mar. 1

11. 0

27.52.42

12.23.40

2

8. 0

28.12.48

12.19.38

5

11.30

29.18. 0

12.03.16

7

9.30

n 0. 4. 0

11.57. 0

9

8.30

0.43. 4

11.45.52

These observations were made by a telescope of 7 feet, with a microme¬
ter and threads placed in the focus of the telescope; by Avhich instruments
we determined the positions both of the fixed stars among themselves, and
of the comet in respect of the fixed stars. Let A represent the star of the
fourth magnitude in the left heel of Perseus (Bayer’s o), B the following
star of the third magnitude in the left foot (Bayer’s C), C a star of the
sixth magnitude (Bayer’s ?i) in the heel of the same foot, and 1), E, F, G,
H, I, K, L, M, N, O, Z, «, 1 3, y, d, other smaller stars in the same foot;
and let p, P, Q, R, S, T, V, X, represent the places of the comet in the
observations above set down ; and, reckoning the distance AB of SO* parts,
AC was 52i of those parts; BO, 58*; AD, 57*; BD, S2*; CD, 23|:
AE, 29}; CE, 57\ ; DE, 49}i; AI, 27 A; BI, 52} ; Cl, 36*; DI,53*;
AK, 38f; BK, 43; OK, 31*; FK, 29; FB, 23; FC, 36}; AH, 18*;
DH, 50}; BN, 46*; CN, 31}; BL, 45*; NL, 31*. HO was to HI
as 7 to 6, and, produced, did pass between the stars D and E, so as the
distance of the star D from this right line was }CD. LM was to LN as
2 to 9, and, produced, did pass through the star H. Thus were the posi¬
tions of the fixed stars determined in respect of one another.

Book III.]

OF NATURAL PHILOSOPHY.

475

Mr. Pound has since observed a second time the positions of thee* fixed
stars amongst themselves, and collected their longitudes and lat* /u&es ac¬
cording to the following table.

The

fixed Their
stars. Longitudes

b 26.41.50
28.40.23
27.58.30
26.27.17
28.28.37
26.56. 8
27.11.45
27.25. 2
27.42. 7

4^6 the mathematical principles [Book III.

The positions of the comet to these fixed stars were observed to be as
follow:

Friday, February 25, O.S. at S^ b . P. M. the distance of the comet in jo
from the star E wai less than fVAE, and greater than jAE, and therefore
nearly equal to AE; and the angle AjdE was a little obtuse, but almost
right. For from A, letting fall a perpendicular on joE, the distance of the
comet from that perpendicular was jjoE.

The same night, at 9i h ., the distance of the comet in P from the star E

was greater than — AE, and less than — AE, and therefore nearly equal

to 27 of AE, or /g AE. But the distance of the comet from the perpen-

dicular let fall from the star A upon the right line PE w T as jPE.

Sunday, February 27, 8 j h . P. M. the distance of the comet in Q, from
the star O was equal to the distance of the stars O and H; and the ri^ht
line Q,0 produced passed between the stars K and B. I could not, by
reason of intervening clouds, determine the position of the star to greater
accuracy.

Tuesday, March 1, ll h . P. M. the comet in R lay exactly in a line be- •
tween the stars K and C, so as the part CR of the right line CRK was a
little greater than ^CK, and a little less than -’CK + {OR, and therefore
= *CK + T V CR, or ifCK.

Wednesday, March 2, S h . P. M. the distance of the comet in S from the
star C was nearly £FC; the distance of the star F from the right line CS
produced was ^V^C; and the distance of the star B from the same right
line was five times greater than the distance of the star F; and the right
line NS produced passed between the stars H and I five or six times nearer
to the star H than to the star I.

Saturday, March 5, ll^ h . P. M. when the comet was in T, the right line
MT was equal to |ML, and the right line LT produced passed between B
and F four or five times nearer to F than to B, cutting off from BF a fifth
or sixth part thereof towards F: and MT produced passed on the outside
of the space BF towards the star B four times nearer to the star B than
to the star F. M was a very small star, scarcely to be seen by the tele¬
scope; but the star L was greater, and of about the eighth magnitude.

Monday, March 7, 9^ h . P. M. the comet being in Y, the right line Va
produced did pass between B and F, cutting off, from BF towards F, T V of
BF, and was to the right line Yj3 as 5 to 4. And the distance of the comet
from the right line a(3 was |Y 3.

Wednesday, March 9, 8 ^ h . P. M. the comet being in X, the right line
yX was equal to |yd; and the perpendicular let fall from the star 6 upon
the right yX was § of yd.

The same night, at 12 b . the comet being in Y, the right line yY was

OF NATURAL PHILOSOPHY.

477

Book III.]

equal to i of yd, or a little less, as perhaps T \ of yd; and a perpendicular
let fall from the star d on the right line yY was equal to about } or } yd.
But the comet being then extremely near the horizon, was scarcely discern¬
ible, and therefore its place could not be determined with that certainty as
in the foregoing observations.

Prom these observations, by constructions of figures and calculations, I
deduced the longitudes and latitudes of the comet; and Mr. Pound, by
correcting the places of the fixed stars, hath determined more correctly the
places of the comet, which correct places are set down above. Though my
micrometer was none of the best, yet the errors in longitude and latitude
(as derived from my observations) scarcely exceed one minute. The comet
(according to my observations), about the end of its motion, bearan -boline
sensibly towards the north, from the parallel which it described about the
end of February.

Now, in order to determine the orbit of the comet out of the observations
above described, I selected those three which Flamsted made, Dec. 21, Jan.
5, and Jan. 25; from which I found of 9S42,1 parts, and Yt of 455
such as the semi-diameter of the or bis magnus contains 10000. Then for
the first observation, assuming cf 5657 of those parts, 1 found SB 9747,
BE for the first time 412, 9503, iX 413, BE for the second time 421,

OD 10186, X 8528,4, PM S450, MN S475, NP 25; from whence, by the
second operation, I collected the distance tb 5640; and by this operation 1
at last deduced the distances TX 4775 and rZ 11322. From which, lim¬
iting the orbit, I found its descending node in 25, and ascending node in V?
1° 53'; the inclination of its plane to the plane of the ecliptic 61° 20^ ,
the vertex thereof (or the perihelion of the comet) distant from the node
8 ° 38', and in t 27° 43', with latitude 7° 34' south; its latus rectum
236,8; and the diurnal area described by a radius drawn to the sun 935S5,
supposing the square of the semi-diameter of the or bis magnus 10U000000;
that the comet in this orbit moved directly according to the order of the
signs, and on Dec. S d . 00\ 04' P. M was in the vertex or perihelion of its
orbit. All which I determined by scale and compass, and the chords of
angles, taken from the table of natural sines, in a pretty large figure, in
which, to wit, the radius of the orbis magnus (consisting of 10000 parts)
wa3 equal to 16|- inches of an English foot.

Lastly, in order to discover whether the comet did truly move in the
orbit so determined, l investigated its places in this orbit partly by arith¬
metical operations, and partly by scale and compass, to the times of gome
of the observations, as may be seen in the following table:—

478

THE MATHEMATICAL PRINCIPLES

[Book III.

The Comet’s ]

Dist.

from

sun.

Longitude

computed.

Latitud.

compu¬

ted.

Longitude

observed.

Latitude

observed

Dif

Lo.

Dif.

Lat.

Dec. 12
29

j Feb. 5
| Mar. 5

2792

8403

16669

121737

V? 6 3 .32'

X 13.131
a 17.00°
29 .19f

8 M8i

1 28. Off
15. 29|
12. 4

V? 6° 311
X 13 .111
» 16 .59*
29 ,20|

8°.26

28 .10 T V
15 .27 f
12 . 34

+ 1
+2
+0
-1

- 74

-10 a

+ 2i

+ 4

But afterwards Dr. Halley did determine the orbit to a greater accu¬
racy by an arithmetical calculus than could be done by linear descriptions;
and, retaining the place of the nodes in 25 and Y? 1° 53', and the inclina¬
tion of the plane of the orbit to the ecliptic 61° 20i', as well as the time
of the comet’s being in perihelio, Dec. S d . 00 h . 04', he found the distance
of the perihelion from the ascending node measured in the comet’s orbit
9° 20', and the latus rectum of the parabola 2430 parts, supposing the
mean distance of the sun from the earth to be 100000 parts; and from
these data, by an accurate arithmetical calculus, he computed the places
of the comet to the times of the observations as follows:—

The Comet’s

Dist from

Longitude

Latitude

Errors in

True time.

the sun.

• ompiited.

computed.

Long.

Lat.

cl h. ' "

u t //

0 t n

/ //

r it

Dec. 12. 4.46.

28028

Y? 6 29 25

8.26. 0 bor.

— 3. 5

— 2. 0

21. 6.37.

61076

7X 5. 6.30

21.43.20

— 1.42

+|7

24. 6.18.

70008

18.48.20

25.22.40

— 1. 3

— 0.25

26. 5.20.

75576

28.22.45

27. 1.36

— 1.28

+ 0.44

29. 8. 3.

84021

X 13.12.40

28.10.10

+ 1.59

+ 0.12

30. 8.10.

86661

17.40. 5

28.11.20

+ 1.45

— 0.33

Jan. 5. 6. l.£

101440

T 8.49.49

26.15.15

+ 0.56

+ 0. 8

9. 7. 0.

110959

18.44.36

24.12.54

+ 0.32

+ 0.58

10. 6. 6.

113162

20.41. 0

23.44.10

+ 0.10

-f 0.18

13. 7. 9.

120000

26. 0.21

22.17.30

4- 0.33

+ 0. 2

25. 7.59.

145370

b 9.33.40

17.57.55

— 1.20

+ 1.25

30. 8.22.

155303

13.17.41

16.42. 7

— 2.10

— 0.11

Feb. 2. 6.35.

160951

15.11.11

16. 4.15

— 2.42

+ 0.14

5. 7. 4 .h

'166686

16.58.55

15.29.13

— 0.41

+ 2. 0

25. 8.41.

202570

26.15.46

12.48. 0

— 2.49

4 - 1.10

Mir. 5.11.39.

216205 j

29.18.35

12. 5.40

+ 0.35

+ 2.14

This comet also appeared in the November before, and at Coburg, in
Saxony, was observed by Mr. Gottfried Kirch, on the 4th of that month, on
the 6th and 11th O. S.; from its positions to the nearest fixed stars observed
with sufficient accuracy, sometimes with a two feet, and sometimes with a
ten feet telescope; from the difference of longitudes of Coburg and Lon¬
don, 11°; and from the places of the fixed stars observed by Mr. Pounds
Dr. Halley has determined the places of the comet as follows :—

OF NATURAL PHILOSOPHY.

479

Book III.]

Nov. 3, 17 h . 2', apparent time at London, the comet was in fl 29 deg.
51', with 1 deg. 17' 45" latitude north.

November 5. 15 h . 58' the comet was in W 3° 23', with 1 ° 6 ' nortl. lat.

November 10, 16 h . 31', the comet was equally distant from two stars in
£1, which are o and t in Bayer ; but it had not quite touched the right
line that joins them, but was very little distant from it. In Flamstecfs
catalogue this star o was then in W 14° 15', with 1 deg. 41' lat. north
nearly, and r in 17° 3^' with 0 deg. 34' lat. south; and the middle
point between those stars wasJD? 15° 39^', with 0° 33^' lat. north. J ,et
the distance of the comet from that right line be about 10 ' or 12 ': and
the difference of the longitude of the comet and that middle point will be
7'; and the difference of the latitude nearly 7|'; and thence it follows
that the comet was in W 15 3 32', with about 26' lat. north.

The f rst observation from the position of the comet with respect tr
certain small fixed stars had all the exactness that could be desired; the
second also was accurate enough. In the third observation, which was the
least accurate, there might be an error of 6 or 7 minutes, but hardly
greater. The longitude of the comet, as found in the first and most
accurate observation, being computed in the aforesaid parabolic orbit,
comes out £l 29° 30' 22", its latitude north 1° 25' 7", and its distance
from the sun 115546.

Moreover, Dr. Halley, observing that a remarkable comet had appeared
four times at equal intervals of 575 years (that is, in the month of Sep¬
tember after Jnliiis Ccesar was killed; A n. Chr. 531, in the consulate of
Lampadins and Orestes; An. Chr. 1106, in the month of February;
and at the end of the year 1680; and that with a long and remarkable
tail, except when it was seen after CcesaFs death, at which time, by reason
of the inconvenient situation of the earth, the tail was not so conspicuous),
set himself to find out an elliptic orbit whose greater axis should be
1382957 parts, the mean distance of the earth from the sun containing
10000 such; in which orbit a comet might revolve in 57 5 years; and,
placing the ascending node in 25 2° 2', the inclination of the plane of the
orbit to the plane of the ecliptic in an angle of 61° 6 ' 4S", the perihelion
of the comet in this plane in 4 22° 44' 25", the equal time of the perihe¬
lion December 7 d . 23 h . 9', the distance of the perihelion from the ascend¬
ing node in the plane of the ecliptic 9^ 17' 35", and its conjugate axis
1S4S 1,2, he computed the motions of the comet in this elliptic orbit. The
places of the comet, as deduced from the observations, and as arising from
computation made in this orbit, may be seen in the following table.

4S0

THE MATHEMATICAL PRINCIPLES

[Book Ill

True time.

Longitude

observed.

Latitude

North

obs.

Longitude

comp.

Latitude

computed.

Erro

Long.

rs in
Lat.

<!. h. '

0 / n

0 t n

v 1 ir

0 1 n

1 «

> 1

Nov. 3.16.47

a

29.51. 0

1.17.45

£1 29.51.22

1.17.32 N

+ 0.22

— 0.13

5.15.37

n

3.23. 0

1. 6. 0

n

3.24.32

1. 6. 9

+ 1.32

+ 0. 9

10.16.18

15.32. 0

0.27. 0

15.33. 2

0 25. 7

+ 1. 2

— 1.53

16.17.00

8.16.45

0.53. 7 S

18.21.34

18.52.15

1.26.54

20.17. 0

28.10.36

1.53 35

. 23.17. 5

ttl 13.22.42

2 29. 0

Dec. 12. 4.46

V?

6.32.30

8.28. 0

V?

6.31.20

8.29. 6 N

— 110

+ 1.6

21. 6.37

5. 8.12

21.42.13

/VV

5. 6.14

21.44.42

— 1.58

+ 2.29

24. 6.18

18.49.23

25.23. 5

18.47.30

25 23 35

— 1.53

+ 0.30

26. 5.21

28.24,13

27. 0.52

28.21.42

27. 2. 1

— 2.31

+ 1. 9

29. 8. 3

X 13.10.41

28. 9.58

X 13.11.14

28.10.38

+ 0.33

+ 0.40

30. 8.10

17.38. 0

28.11.53

17.38 27

28.11.37

+ 0. 7

— 0.16

Jan. 5. 6. l£

op

8.48.53

26.15. 7

op

8.48.51

26.14.57

— 0. 2

— 0.10

9. 7. 1

18.44. 4

24 11.56

18.43.51

24.12.17

— 0.13

+ 0.21

10. 6. 6

20.40.50

23 43.32

20.40.23

23.43.25

— 0.27

— 0. 7

13. 7. 9

25.59.48

22 17.28

26. 0. 8

22.16 32

+ 0.20

— 0.56

25. 7.59

9.35. 0

17.56.30

b

9.34.11

17.56. 6

— 0.49

— 0.24

30. 8.22

13.19.51

16.42.18

13.18.28

16.40. 5

— 1.23

— 2.13

Feb. 2. 6.35

15.13.53

16. 4. 1

15.11.59

16. 2 17

— 1.54

— 1.54

5. 7. 4 h

16.59. 6

15.27. 3

16.59.17

15.27. 0

+ 0.11

— 0. 3

25. 8.41

26.18.35

12.46.46

26 16.59

12.45.22

— 1.36

— 1.24

Mar. 1.11.10

27.52.42

12.23 40

27.51.47

12.22.28

— 0.55

— 1.12

5.11.39

29.18. 0

12. 3.16

29 20.11

12. 2 50

+ 2.11

— 0.26

9. 8.38

n

0.43. 4

11.45.52

n

0.42.43

11.45.35

— 0.21

— 0.17

The observations of this comet from the beginning to the end agree at
perfectly with the motion of the comet in the orbit just now described aa
the motions of the planets do with the theories from whence they are cal¬
culated ; and by this agreement plainly evince that it was one and the
same comet that appeared all that time, and also that the orbit of that
comet is here rightly defined.

In the foregoing table we have omitted the observations of Nov. 16,
18, 20. and 23, as not sufficiently accurate, for at those times several per¬
sons had observed the comet. Nov. 17, O. S. Ponthceus and his compan¬
ions, at 6 h . in the morning at Rome (that is, 5 h . 10' at London), by threads
directed to the fixed stars, observed the comet in = 2 = 8° 30', with latitude 0°
40' south. Their observations may be seen in a treatise which Ponthceus
published concerning this comet. Cellius , who was present, and commu¬
nicated his observations in a letter to Cassini , saw the comet at the same
hour in = 2 = 8° 30', with latitude 0° 30' south. It was likewise seen by
Galletius at the same hour at Avignon (that is, at 5 h . 42' morning at
London) in = 2 = 8° without latitude. But by the theory the comet was at
that time in = 2 = 8° 16' 45", and its latitude was 0° 53' 7" south.

Nov. 18, at 6 h . 30' in the morning at Rome (that is, at 5 h . 40' at Lon¬
don), Ponthceus observed the comet in ^ 13° 30', with latitude 1° 20

OF NATURAL PHII OSOPHY.

481

Book III.]

south; and Cellius in ^ 13° 30', with latitude 1° 00' south. But at 5 h .
30' in the morning at Avignon, Galletias saw it in — 13° 00', with lati¬
tude 1° 00' south. In the University of La Fleche , in France , at 5 h . in
the morning (that is, at 5 h . 9' at London ), it was seen by P. An go, in the
middle between two small stars, one of which is the middle of the three
which lie in a right line in the southern hand of Virgo, Bayers ip ; and
the other is the outmost of the wing, Bayer s 6. Whence the comet was
then in ^ 12° 46' with latitude 50' south. And I was informed by Dr.
Halley , that on the same day at Boston in New England, in the latitude
of 42^ deg. at 5 h . in the morning (that is, at 9 h . 44' in the morning at
London ), the comet was seen near === 14°, with latitude 1° 30' south.

Nov. 19, at 4| h . at Cambridge, the comet (by the observation of a
young man) w r as distant from Spica about 2° towards the north west.
Now the spike was at that time in ^ 19° 23' 47", with latitude 2° 1' 59"
south. The same day, at 5 h . in the morning, at Boston in New England,
the comet was distant from Spica 1°, with the difference of 40' in lati¬
tude. The same day, in the island of Jamaica , it was about 1° distant
from Spica W. The same day, Mr. Arthur Storer, at the river Patuxent ,
near Hunting Creek, in Maryland , in the confines of Virginia , in lat.
38i°, at 5 in the morning (that is, at 10 h . at London), saw the comet
above Spica W, and very nearly joined with it, the distance between them
being about £ of one deg. And from these observations compared. I con¬
clude, that at 9 h . 44' at London, the comet was in 18° 50', with about
1 ° 25' latitude south. Now by the theory the comet was at that time in
^ 18° 52' 15", with 1° 26' 54" lat. south.

Nov. 20, Montenari, professor of astronomy at Padua, at 6 h . in the
morning at Venice (that is, 5 h . 10' at London), saw the comet in ^ 23°,
with latitude 1° 30' south. The same day, at Boston, it was distant from
Spica W by about 4° of longitude east, and therefore was in =£= 23° 24'
nearly.

Nov. 21, Ponthceus and his companions, at 7£ h . in the morning, ob¬
served the comet in ^ 27° 50', with latitude 1° 16' south; Cellius, in ^
28° ; P. An go at 5 h . in the morning, in ^ 27° 45'; Montenari in ^
27° 51'. The same day, in the island of Jamaica, it was seen near the
beginning of ^1, and of about the same latitude with Spica % that is, 2°
2'. The same day, at 5 h . morning, at Ballasore, in the East Indies (that
is, at ll h . 20' of the night preceding at London), the distance of the
comet from Spica ty was taken 7° 35' to the east. It was in a right line
between the spike and the balance, and therefore was then in 26° 58',
with about 1° 11' lat. south; and after 5 h . 40' (that is, at 5 h . morning at
London), it was in === 28° 12'. with 1° 16' lat. south. Now by the theory
She comet was then in ^ 28° 10' 36", with 1° 53' 35" lat. south.

Nov. 22, the comet was seen by Montenari in ^1. 2° 33'; but at Boston

31

482

THE MATHEMATICAL PRINCIPLES

[Book 1IL

in New England, it was found in about 3°, and with almost the same
latitude as before, that is, 1° 30'. The same day, at 5 h . morning at
Ballasore, the comet was observed in 3% 1° 50'; and therefore at 5 h . morn¬
ing at London , the comet was in ^1 3° 5' nearly. The same day, at 6^ h .
in the morning at London , Ur. Hook observed it in about nt 3° 30', and
that in the right line which passeth through Spica W and Cor Leonis ;
not, indeed, exactly, but deviating a little from that line towards the
north. Mon ten an likewise observed, that this day, and some days after,
a right line drawn from the comet through >S 'pica passed by the south
side of Cor Leonis at a very small distance therefrom. The right line
through Cor Leonis and Spica W did cut the ecliptic in W 3° 4(5' at an
angle of 2° 51'; and if the comet had been in this line and in 3°, its
latitude would have been 2° 26'; but since Hook and Montenari agree
that the comet was at some small distance from this line towards the
north, its latitude must have been something less. On the 20th, by the
observation of Montenari, its latitude was almost the same with that of
Spica •).?!, that is, about 1° 30'. But by the agreement of Hook, Monte¬
nari, and An go, the latitude was continually increasing, and therefore
must now, on the 22\ be sensibly greater than 1° 30'; and, taking a
mean between the extreme limits but now stated. 2° 2(5' and l c 30', the
latitude will be about 1° 5S'. Hook and Montenari agree that the tail
of the comet was directed towards Spica W, declining a little from that
star towards the south according to Hook, but towards the north according
to Montenari ; and, therefore, that declination was scarcely sensible; and
the tail, lying nearly parallel to the equator, deviated a little from the op¬
position of the sun towards the north.

Nov. 23, O. S. at 5 1 . morning, at Nuremberg (that is, at 4^ h . at Lou¬
don), Mr. Zimmerman saw the comet in 8° S', wfith 2° 31' south lat.
its place being collected by taking its distances from fixed stars.

Nov. 24, before sun-rising, the comet was seen by Montenari in 1U IP 3
52' on the north side of the right line through Cor Leonis and Spica tty,
and therefore its latitude was something less than 2° 38'; and since the
latitude, as we said, by the concurring observations of Montenari, An go,
and Hook, was continually increasing, therefore, it was now, on the 24th,
something greater than 1° 58' | and, taking the mean quantity, may be
reckoned 2° IS”, without any considerable error. Ponthrens and Galletms
will have it that the latitude was now decreasing; and Cellius, and the
observer in New England, that it continued the same, viz., of about 1°,
or 1^°. The observations of Ponthceus and Cellius are more rude, espe¬
cially those which were made by taking the azimuths and altitudes; as
are also the observations of Galletius. Those are better which were
made by taking the position of the comet to the fixed stars by Montenari^
Hook , Ango, and the observer in New England, and sometimes by

Book III J

OF NATURAL PHILOSOPHY.

4S3

Ponlhceus and Cellius. The same day, at 5 h . morning, at Ballasore , the
comet was observed in W 11° 45'; and, therefore, at 5 h . morning at Lon¬
don, was in 13° nearly. And, by the theory, the comet was at that
time in 1% 13° 22' 42".

Nov. 25, before sunrise, Montenari observed the comet in ir L 17£
nearly ; and Cellius observed at the same time that the comet was in a
right line between the bright* star in the right thigh of Yirgo and the
southern scale of Libra; and this right line cuts the comets way in HI
LS° 36'. And, by the theory, the comet was in ^1 1S^° nearly.

From all this it is plain that these observations agree with the theory,
so far as they agree with one another; and by this agreement it is made
clear that it was one and the same comet that appeared all the time from
Nov. 4 to Mar. 9. The path of this comet did twice cut the plane of the
ecliptic, and therefore was not a right line. It did cut the ecliptic not in
opposite parts of the heavens, but in the end of Yirgo and beginning of
Capricorn, including an arc of about 98°; and therefore the way of the
comet did very much deviate from the path of a great circle; for in the
month of Nov. it declined at least 3° from the ecliptic towards the south ;
and in the month of Dec. following it declined 29° from the ecliptic to¬
wards the north ; the two parts of the orbit in which the comet descended
towards the sun, and ascended again from the sun, declining one from the
other by an apparent angle of above 30°, as observed by Montenari. This
comet travelled over 9 signs, to wit, from the last deg. of £1 to the begin¬
ning of n, beside the sign of £1, through which it passed before it began
to be seen; and there is no other theory by which a comet can go over so
great a part of the heavens with a regular motion. The motion of this
comet was very unequable; for about the 20th of Nov. it described about
5° a day. Then its motion being retarded between Nov. 26 and Dec.
12, to wit, in the space of 15i days, it described only 40°* But the mo¬
tion thereof being afterwards accelerated, it described near 5° a day, till
its motion began to be again retarded. And the theory which justly cor¬
responds with a motion so unequable, and through so great a part of the
heavens, which observes the same laws with the theory of the planets, and
which accurately agrees with accurate astronomical observations, cannot
be otherwise than true.

And, thinking it would not be improper, 1 have given a true representa¬
tion of the orbit which this comet described, and of the • tail which it
emitted in several places, in the annexed figure; protracted in the plane of
the trajectory. In this scheme ABC represents the trajectory of the comet,
D the sun DE the axis of the trajectory, DF the line of the nodes, GH
the intersection of the sphere of the orbis mag mis with the plane of the
trajectory. I the place of the comet Nov. 4, Ann. 16S0; K the place of the
same Nor. 11; L the place of the same Nov. 19; M its place Dec. 12; N

THE MATHEMATICAL PRINCIPLES

IBook III.

184

its place Dec. 21; O its place Dec. 29; P its place Jan. 5 following; Q
its place Jan. 25; R its place Feb. 5 ; S its place Feb . 25; T its place
March 5 ; and V its place March 9. In determining the length of the
tail, I made the following observations.

Nov. 4 and 6, the tail did not appear; Nov. 11, the tail just begun to
shew itself, but did not appear above 4 deg. long through a 10 feet tele¬
scope; Nov. 17, the tail was seen by Ponthceus more than 15° long; Nov.
18, in Neic-England, the tail appeared 30° long, and directly opposite to
the sun, extending itself to the planet Mars, which was then in W, 9° 54 :
Nov. 19. in Maryland , the tail was found 15° or 20 5 Ions:; Dec. 10 (by

Book III.] of natural philosophy. 4S5

the observation of Mr. Flamsted ), the tail passed through the middle of
the distance intercepted between the tail of the Serpent of Ophiuchus and
the star <$ in the south wing of Aquila , and did terminate near the stars
A, w, b, in Bayeds tables. Therefore the end of the tail was in V ? 19 1 ®,
with latitude about 34±° north; Dec 11 , it ascended to the head of Sagit-
ta ( Bayer's a, (3), terminating in Y? 26° 43', with latitude 38° 34' north;
Dec. 12, it passed through the middle of Sagitta , nor did it reach much
farther; terminating in ~ 4°, with latitude 4 2\° north nearly. But these
things are to be understood of the length of the brighter part of the tail;
for with a more faint light, observed, too, perhaps, in a serener sky, at
Rome, Dec. 12, 5 h . 40', by the observation of Ponthceus , the tail arose to
1CP above the rump of the Swan, and the side thereof towards the west
and towards the north was 45' distant from this star. But about that time
the tail was 3° broad towards the upper end; and therefore the middle
thereof was 2° 15' distant from that star towards the south, and the upper
end was X in 22 °, with latitude 61° north; and thence the tail was about
70° long; Dec. 21, it extended almost to Cassiopeia’s chair, equally dis¬
tant from j3 and from Schedir , so as its distance from either of the two
was equal to the distance of the one from the other, and therefore did ter¬
minate in T 24°, with latitude 47^° ; Dec. 29, it reached to a contact with
Scheat on its left, and exactly tilled up the space between the two stars in
the northern foot of Andromeda , being 54° in length; and therefore ter¬
minated in 3 19°, with 35° of latitude; Jan 5, it touched the star rc in
the breast of Andromeda, on its right side, and the star /1 of the girdle on
its left; and, according to our observations, was 40° long; but it was
curved, and the convex side thereof lay to the south; and near the head of
the comet it made an angle of 4° with the circle which passed through the
sun and the comet’s head; but towards the other end it was inclined to
that circle in an angle of about 10° or 11 °; and the chord of the tail con¬
tained with that circle an angle of 8 °. Jan. 13, the tail terminated be¬
tween Alamech and Algol , with a light that was sensible enough ; but
with a faint light it ended over against the star n in Perseus's side. The
distance of the end of the tail from the circle passing through the sun and
the comet was 3° 50'; and the inclination of the chord of the tail to that
circle was 8 ^°. Jan. 25 and 26, it shone with a faint light to the length
of 6 ° or 7°; and for a night or two after, when there was a very clear sky,
it extended to the length of 12 °, or something more, with a light that was
very faint and very hardly to be seen ; but the axis thereof was exactly di¬
rected to the bright star in the eastern shoulder of Auriga , and therefore
deviated from the opposition of the sun towards the north by an angle of
10°. Lastly, Feb. 10, with a telescope I observed the tail 2° long ; for that
fainter light which I spoke of did not appear through the glasses. But
Ponthceus writes, that, on Feb. 7, he saw the tail 12° long. Feb. 25, the
comet was without a tail, and so continued till it disappeared

486

THE MATHEMATICAL PRINCIPLES

[Book III.

Now if one reflects upon the orbit described, and duly considers the other
appearances of this comet, he will be easily satisfied that the bodies ot
comets are solid, compact, fixed, and durable, like the bodies of the planets;
for if they were nothing else but the vapours or exhalations of the earth, of
the sun, and other planets, this comet, in its passage by the neighbourhood
of the sun, would have been immediately dissipated; for the heat of the
sun is as the density of its rays, that is, reciprocally as the square of the
distance of the places from the sun. 'Therefore, since on Dec. 8, when the
comet was in its perihelion, the distance thereof from the centre of the sun
was to the distance of the earth from the same as about 6 to 1000, the
sun’s heat on the comet was at that time to the heat of the summer-sun
with us as 1000000 to 36, or as 28000 to 1. But the heat of boiling
water is about 3 times greater than the heat which dry earth acquires from
the summer-sun, as 1 have tried; and the heat of red-hot iron (if my con¬
jecture is right) is about three or four times greater than the heat of boil¬
ing water. And therefore the heat which dry earth on the comet, while in
its perihelion, might have conceived from the rays of the sun, was about
2000 times greater than the heat of red -hot iron. But by so fierce a heat,
vapours and exhalations, and every volatile matter, must have been imme¬
diately consumed and dissipated.

'This comet, therefore, must have conceived an immense heat from the
sun, and retained that heat for an exceeding long time; for a globe of iron
of an inch in diameter, exposed red-hot to the open air, will scarcely lose
all its heat in an hour’s time; but a greater globe would retain its heat
longer in the proportion of its diameter, because the surface (in proportion
to which it is cooled by the contact of the ambient air) is in that proportion
less in respect of the quantity of the included hot matter; and therefore a
globe of red hot iron equal to our earth, that is, about 40000000 feet in
diameter, would scarcely cool in an equal number of days, or in above
50000 years. But I suspect that the duration of heat may, on account of
some latent causes, increase in a yet less proportion than that of the
diameter; and I should be glad that the true proportion was investigated
by experiments.

It is farther to be observed, that the comet in the month of December .
just after it had been heated by the sun, did emit a much longer tail, and
much more splendid, than in the month of November before, when it had
not yet arrived at its perihelion; and, universally, the greatest and most
fulgent tails always arise from comets immediately .fter their passing by
the neighbourhood of the sun. Therefore the heat received by the comet
conduces to the greatness of the tail: from whence, I thick I may infer,
that the tail is nothing else but a very fine vapour, which the head or
nucleus of the comet emits by its heat.

Hut we have had three several opinions about the tails of comets; for

OF NATURAL PHILOSOPHY.

4S?

Book III.]

some will have it that they are nothing else but the beams of the sun’s
light transmitted through the comets’ heads, which they suppose to be
transparent; others, that they proceed from the refraction which light suf¬
fers in passing from the comet’s head to the earth: and, lastly, others, thac
they are a sort of clouds or vapour constantly rising from the comets’ heads,
and tending towards the parts opposite to the sun. The first is the opin¬
ion of such as are yet unacquainted with optics; for the beams of the sun
are seen in a darkened room only in consequence of the light that is re-
riected from them by the little particles of dust and smoke which are
always flying about in the air; and, for that reason, in air impregnated
with thick smoke, those beams appear with great brightness, and move the
sense vigorously; in a yet finer air they appear more faint, and are less
easily discerned ; but in the heavens, where there is no matter to reflect
the light., they can never be seen at all. Light is not seen as it is in the
beam, but as it is thence reflected to our eyes; for vision can be no other¬
wise produced than by rays falling upon the eyes ; and, therefore, there
mast be some reflecting matter in those parts where the tails of the comets
are seen : for otherwise, since all the celestial spaces are equally illumin¬
ated by the sun’s light, no part of the heavens could appear with more
splendor than another. The second opinion is liable to many difficulties.
'The tails of comets are never seen variegated with those colours which
commonly are inseparable from refraction ; and the distinct transmission
of the light of the fixed stars and planets to us is a demonstration that
the aether or celestial medium is not endowed with any refractive power :
for as to what is alleged, that the fixed stars have been sometimes seen by
the Egyptians environed with a Coma or CapitHtium , because that has
but rarely happened, it is rather to be ascribed to' a casual refraction of
clouds; and so the radiation and scintillation of the fixed stars to tin
refractions both of the eyes and air; for upon laying a telescope to the
eye, those radiations and scintillations immediately disappear. By the trem¬
ulous agitation of the air and ascending vapours, it happens that the rays of
light are alternately turned aside from the narrow space of the pupil of the
eye; but no such thing can have place in the much wider aperture of the ob¬
ject-glass of a telescope ; and hence it is that a scintillation is occasioned in
the former case, which ceases in the latter ; and this cessation in the latter
case is a demonstration of the regular transmission of light through the
heavens, without any sensible refraction. But, to obviate an objection
that may be made from the appearing of no tail in such comets as shine
but with a faint light, as if the secondary rays were then too weak to af¬
fect the eyes, and for that reason it is that the tails of the fixed stars do
not appear, we are to consider, that by the means of telescopes the light of
the fixed stars may be augmented above an hundred fold, and yet no tails
are seen ; that the light of the planets is yet more copious without anv

188

THE MATHEMATICAL PRINCIPLES

[Book III.

-ail; but that comets are seen sometimes with huge tails, when the light
of their heads is but faint and dull. For so it happened in the comet of
the year 16SO, when in the month of December it was scarcely equal in
light to the stars of the second magnitude, and yet emitted a notable tail,
extending to the length of 40°, 50 3 , 60°, or 70°, and upwards; and after¬
wards, on the 27th and 28th of January, when the head appeared but as
a star of the 7th magnitude, yet the tail (as we said above), with a light
that was sensible enough, though faint, was stretched out to 6 or 7 degrees
in length, and with a languishing light that was more difficultly seen, even
to 12°, and upwards. But on the Oth and 10th of February, when to the
naked eye the head appeared no more, through a telescope I viewed the
tail of 2° in length. But farther; if the tail was owing to the refrac¬
tion of the celestial matter, and did deviate from the opposition of the
sun, according to the figure of the heavens, that deviation in the same
places of the heavens should be always directed towards the same parts.
Bu rt e comet of the year 1680, December 28 d . S^ h . P. M. at London , was
seen in X 8° 41', with la'itude north 2S°.6'; while the sun was in Y? 1S°
26'. And the comet of the year 1577, December 29 d . was in X 8° 41',
with latitude north 28 40', and the si n, as before, in about 18° 26'.
[n both cases the situation of the earth was the same, and the comet ap¬
peared in the same place of the heavens; yet in the former case the tail
of the comet (as well by my observations as by the observations of others)
deviated from the opposition of the sun towards the north by an angle of
4 ^ degrees ; whereas in the latter there was (according to the observations
of Tycho) a deviation of 21 degrees towards the south. The refraction,
therefore, of the heavens being thus disproved, it remains that th e phe¬
nomena of the tails of comets must be derived from some reflecting matter.

And that the tails of comets do arise from their heads, and tend towards
the parts opposite to the sun, is farther confirmed from the laws which
the tails observe. As that, lying in the planes of the comets’ orbits
which pass through the sun, they constantly deviate from the opposition
of the sun towards the parts which the comets’ heads in their progress
along these orbits have left. That to a spectator, placed in those planes,
they appear in the parts directly opposite to the sun ; but, as the spectator
recedes from th >se planes, their deviation begins to appear, and daily be¬
comes greater. That the deviation, cceteris paribus , appears less when
the tail is more oblique to the orbit of the comet, as well as when the
head of the comet approaches nearer to the sun, especially if the angle of
deviation is estimated near the head of the comet. That the tails which
have no deviation appear straight, but the tails which deviate are like¬
wise bended into a certain curvature. That this curvature is greater when
the deviation is greater; and is more sensible when the tail, cater is pari¬
bus is longer ; for in the shorter tails the curvature is hardly to be p jr-

Hook 111.] of natural philosophy. 489

ceived. That the angle of deviation is less near the comet’s head, but
o-reater towards the other end of the tail; and that because the convex
side of the tail regards the parts from which the deviation is made, and
which lie in a right line drawn out infinitely from the sun through the
comet’s head. And that the tails that are long and broad, and shine with
a stronger light, appear more resplendent and more exactly defined on the
convex than on the concave side. Upon which accounts it is plain that
the phcenorneti a of the tails of comets depend upon the motions of their
heads, and by no means upon the places of the heavens in which their
heads are seen ; and that, therefore, the tails of comets do not proceed from
the refraction of the heavens, but from their own heads, which furnish the
matter that forms the tail. For, as in our air, the smoke of a heated body
ascends either perpendicularly if the body is at rest, or obliquely if the
body is moved obliquely, so in the heavens, where all bodies gravitate to¬
wards the sun, smoke and vapour must (as we have already said) ascend
from the sun, and either rise perpendicularly if the smoking body is at
rest, or obliquely if the body, in all the progress of its motion, is always
leaving those places from which the upper or higher parts of the vapour
had risen before ; and that obliquity will be least where the vapour ascends
with most velocity, to wit, near the smoking body, when that is near the
sun. But, because the obliquity varies, the column of vapour will be iri-
curvated; and because the vapour in the preceding sides is something more
recent, that is, has ascended something more , late from the body, it will
therefore be something more dense on that side, and must on that account
reflect more light, as well as be better defined. I add nothing concerning
the sudden uncertain agitation of the tails of comets, and their irregular
figures, which authors sometimes describe, because they may arise from the
mutations of our air, and the motions of our clouds, in part obscuring
those tails; or, perhaps, from parts of the Via LacAea , which might have
been confounded with and mistaken for parts of the tails of the comets as
they passed by.

But that the atmospheres of comets may furnish a supply of vapour
great enough to fill so immense spaces, we may easily understand from the
rari ty of our own air; for the air near the surface of our earth possesses
a space S50 times greater than water of the same weight: and therefore
a cylinder of air S50 feet high is of equal weight with a cylinder of water
of the same breadth, and but one foot high. But a cylinder of air reach¬
ing to the top of the atmosphere is of equal weight with a cylinder of
water about 33 feet high : and, therefore, if from the whole cylinder of
air the lower part of 850 feet high is taken away, the remaining upper
part will be of equal weight with a cylinder of water 32 feet high: and
from thence (and by the hypothesis, confirmed by many experiments, that
the compression of air is as the weight of the incumbent atmosphere, and

400

THE MATHEMATICAL PRINCIPLES

[Book III

that the force of gravity is reciprocally as the square of the distance from
the centre of the earth) raising a calculus, by Cor. Prop. XXII, Book II,
I found, that, at the height of one semi-diameter of the earth, reckoned
from the earth’s surface, the air is more rare than with us in a far greater
proportion than of the whole space within the orb of Saturn to a spherical
space of one inch in diameter; and therefore if a sphere of our air of but
one inch in thickness was equally rarefied with the air at the height of
one semi-diameter of the earth from the earth’s surface, it would fill all
the regions of the planets to the orb of Saturn, and far beyond it. Where¬
fore since the air at greater distances is immensely rarefied, and the coma
or atmosphere of comets is ordinarily about ten times higher, reckoning
from their centres, than the surface of the nucleus, and the tails rise yet
higher, they must therefore be exceedingly rare; and though, on account
of the much thicker atmospheres of comets, and the great gravitation of
their bodies towards the sun, as 'well as of the particles of their air and
vapours mutually one towards another, it may happen that the air in the
celestial spaces and in the tails of comets is not so vastly rarefied, yet
from this computation it is plain that a very small quantity of air and
vapour is abundantly sufficient to produce all the appearances of the tails
of comets ; for that they are, indeed, of a very notable rarity appears from
the shining of the stars through them. The atmosphere of the earth,
illuminated by the sun’s light, though but of a few miles in thickness,
quite obscures and extinguishes the light not only of all the stars, but
even of the moon itself; whereas the smallest stars are seen to shine
through the immense thickness of the tails of comets, likewise illuminated
by the sun, without the least diminution of their splendor. Nor is the
brightness of the tails of most comets ordinarily greater than that of our
air, an inch or two in thickness, reflecting in a darkened room the light of
the sun-beams let in by a hole of the window-shutter.

And we may pretty nearly determine the time spent during the ascent
of the vapour from the comet’s head to the extremity of the tail, by draw¬
ing a right line from the extremity of the tail to the sun, and marking
the place where that right line intersects the comet’s orbit: for the vapour
that is now in the extremity of the tail, if it has ascended in a right line
from the sun, must have begun to rise from the head at the time when the
head was in the point of intersection. It is true, the vapour does not rise
in a right line from the sun, but, retaining the motion which it had from
the comet before its ascent, and compounding that motion with its motion
of ascent, arises obliquely ; and, therefore, the solution of the Problem'will
be more exact, if we draw the line which intersects the orbit parallel to
the length of the tail; or rather (because of the curvilinear motion of the
comet) diverging a little from the line or length of the tail. And by
means of this principle I found that the vapour which, January 25, was

Boo* III.]

OF NATURAL PHILOSOPHY.

491

in the extremity of the tail, had begun to rise from the head before De¬
cember 11, and therefore had spent in its whole ascent 45 days; but that
the whole tail which appeared on December 10 had finished its ascent in
the space of the two days then elapsed from the time of the comet’s being
in its perihelion. The vapour, therefore, about the beginning and in the
neighbourhood of the sun rose with the greatest velocity, and afterwards
continued to ascend with a motion constantly retarded by its own gravity;
and the higher it ascended, the more it added to the length of the tail;
and while the tail continued to be seen, it was made up of almost, all that
vapour which had risen since the time of the comet’s being in its perihe¬
lion ; nor did that part of the vapour which had risen first, and which
formed the extremity of the tail, cease to appear, till its too great dis¬
tance, as well from the sun, from which it received its light, as from our
eyes, rendered it invisible. Whence also it is that the tails of other comets
which are short do not rise from their heads with a swift and continued
motion, and soon after disappear, but are permanent and lasting columns
of vapours and exhalations, which, ascending from the heads with a slow
motion of many days, and partaking of the motion of the heads which
they had from the beginning, continue to go along together wfith them
through the heavens. From whence again we have another argument
proving the celestial spaces to be free, and without resistance, since in
them not only the solid bodies of the planers and comets, but also the ex¬
tremely rare vapours of comets’ tails, maintain their rapid motions with
great freedom, and for an exceeding long time.

Kepler ascribes the ascent of the tails of the comets to the atmospheres
of their heads; and their direction towards the parts opposite to the sun to
the action of the rays of light carrying along with them the matter of the
comets’ tails; and without any great incongruity we may suppose, that, in
so free spaces, so fine a matter as that of the mther may yield to the action
of the rays of the sun’s light, though those rays are not able sensibly to
move the gross substances in our parts, which are clogged with so palpable
a resistance. Another author thinks that there may be a sort of particles
of matter endowed with a principle of levity, as well as others are with a
power of gravity; that the matter of the tails of comets may be of the
former sort, and that its ascent from the sun may be owing to its levity;
but, considering that the gravity of terrestrial bodies is as the matter of
the bodies, and therefore can be neither more nor less in the same quantity
of matter, I am inclined to believe that this ascent may rather proceed from
the rarefaction of the matter of the comets’ tails. The ascent of smoke in
a chimney is owing to the impulse of the air with which it is entangled.
The air rarefied by heat ascends, because its specific gravity is diminished,
and in its ascent carries along with it the smoke with which it is engaged;
xnd why may not the tail of a comet rise from the sun after the same man-

492

THE MATHEMATICAL PRINCIPLES

[Book III.

tier ? For the sun’s rays do not act upon the mediums which they per¬
vade otherwise than by reflection and refraction ; and those reflecting par¬
ticles heated by this action, heat the matter of the aether which is involved
with them. That matter is rarefied by the heat which it acquires, and be¬
cause, by this rarefaction, the specific gravity with which it tended towards
the sun before is diminished, it will ascend therefrom, and carry along with
it the reflecting particles of which the tail of the comet is composed. But
the ascent of the vapours is further promoted by their circumgyration
about the sun, in consequence whereof they endeavour to recede from the
sun, while the sun’s atmosphere and the other matter of the heavens are
either altogether quiescent, or are only moved with a slower circumgyra¬
tion derived from the rotation of the sun. And these are the causes of the
ascent of the tails of the comets in the neighbourhood of the sun, where
their orbits are bent into a greater curvature, and the comets themselves
are plunged into the denser and therefore heavier parts of the sun’s atmos¬
phere : upon which account they do then emit tails of an huge length; for
the tails which then arise, retaining their own proper motion, and in the
mean time gravitating towards the sun, must be revolved in ellipses about
the sun in like manner as the heads are, and by that motion must always
accompany the heads, and freely adhere to them. For , the gravitation ol
the vapours towards the sun can no more force the tails to abandon the
heads, and descend to the sun,‘than the gravitation of the heads can oblige
them to fall from the tails. They must by their common gravity either
fall together towards the sun, or be retarded together in their common as¬
cent therefrom ; and, therefore (whether from the causes already described,
or from any others), the tails and heads of comets may easily acquire and
freely retain any position one to the other, without disturbance or impedi¬
ment from that common gravitation.

The tails, therefore, that rise in the perihelion positions of the comets
will go along with their heads into far remote parts, and together with
the heads will either return again from thence to us, after a long course of
years, or rather will be there rarefied, and by degrees quite vanish away ;
for afterwards, in the descent of the heads towards the sun, new short tails
will be emitted from the heads with a slow motion; and those tails by de¬
grees w r ill be augmented immensely, especially in such comets as in their
perihelion distances descend as low as the sun’s atmosphere; for all vapour
in those free spaces is in a perpetual state of rarefaction and dilatation;
and from hence it is that the tails of all comets are broader at their upper
extremity than near their heads. And it is not unlikely but that the va¬
pour, thus perpetually rarefied and dilated, may be at last dissipated and
scattered through the whole heavens, and by little and little be attracted
towards, the planets by its gravity, and mixed with their atmosphere; for
as the seas are absolutely necessary to the constitution of our earth, that

Book III.]

OF NATURAL PHILOSOPHY.

493

from them, the sun, by its heat, may exhale a sufficient quantity of vapours,
which, being gathered together into clouds, may drop down in rain, for
watering of the earth, and for the production and nourishment of vegeta¬
bles; or, being condensed with cold on the tops of mountains (as some phi¬
losophers with reason judge), may run down in springs and rivers; so for
the conservation of the seas, and fluids of the planets, comets seem to be
required, that, from their exhalations and vapours condensed, the wastes of
the planetary fluids spent upon vegetation and putrefaction, and converted
into dry earth, may be continually supplied and made up; for all vegeta¬
bles entirely derive their growths from fluids, and afterwards, in great
measure, are turned into dry earth by putrefaction: and a sort of slime is
always found to settle at the bottom of putrefied fluids; and hence it is
that the bulk of the solid earth is continually increased; and the fluids, if
they are not supplied from without, must be in a continual decrease,
and quite fail at last. I suspect, moreover, that it is chiefly from the
comets that spirit comes, which is indeed the smalles': but the most subtle
and useful part of our air, and so much required to sustain the life of all
things with us.

The atmospheres of comets, in their descent towards the sun, by running
out into the tails, are spent and diminished, and become narrower, at least
on that side which regards the sun; and in receding from the sun, when
they less run out into the tails, they are again enlarged, if Hevelius has
justly marked their appearances. But they are seen least of all just after
they have been most heated by the sun, and on that account then emit the
longest and most resplendent tails; and, perhaps, at the same time, the
nuclei are environed with a denser and blacker smoke in the lowermost
parts of their atmosphere; for smoke that is raised by a great and intense
heat is commonly the denser and blacker. Thus the head of that comet
which we have been describing, at equal distances both from the sun and
from the earth, appeared darker after it had passed by its perihelion than
it did before; for in the month of December it was commonly compared
with the stars of the third magnitude, but in November with those of the
first or second; and such as saw both appearances have described the first
as of another and greater comet than the second. For, November 19, this
comet appeared to a young man at Cambridge , though with a pale and
dull light, yet equal to Spica Virginis; and at that time it shone with
greater brightness than it did afterwards. And Montenari, November 20,
et. vet. observed it larger than the stars of the first magnitude, its tail
being then 2 degrees long. And Mr. Storer (by letters which have come
into my hands) writes, that in the month of December , when the tail ap¬
peared of the greatest bulk and splendor, the head was but small, and far
less than that which was seen in the month of November before sun-rising;
and, conjecturing at the cause of the appearance, he judged it to proceed

494 THE MATHEMATICAL PRINCIPLES [BOOK J ll

from there being a greater quantity of matter in the head at first, which
was afterwards gradually spent.

And, which farther makes for the same purpose, I find, that the heads of
other comets, which did put forth tails of the greatest bulk and splendor,
have appeared but obscure and small. For in Brazil, March 5, 1668, 7 h .
L\ 3VL, St. N. P. Valentinus Estaucius saw a comet near the horizon, and
towards the south west, with a head so small as scarcely to be discerned,
but with a tail above measure splendid, so that the reflection thereof from
the sea was easily seen by those who stood upon the shore; and it looked
like a fiery beam extended 23° in length from the west to south, almost
parallel to the horizon. But this excessive splendor continued only three
days, decreasing apace afterwards; and while the splendor was decreasing,
the bulk of the tail increased : whence in Portugal it is said to have taken
ap one quarter of the heavens, that is, 45 degrees, extending from west to
iast with a very notable splendor, though the whole tail was not seen in
those parts, because the head was always hid under the horizon: and from
the increase of the bulk and decrease of the splendor of the tail, it appears
that the head was then in its recess from the sun, and had been very near
to it in its perihelion, as the comet of 1680 was. And we read, in the
Saxon Chronicle, of a like comet appearing in the year 1106, the star
whereof was small and obscure (as that of 1680), but the splendour of its
tail was very br ight , and like a huge fiery beam stretched out in a direc¬
tion between the east and north , as Ilevelius has it also from Simeon, the
monk of Durham. This comet appeared in the beginning of February.
about the evening, and towards the south west part of heaven; from
whence, and from the position of the tail, we infer that the head was near
the sun. Matthew Paris says, It was distant from the sun, by about a
cubit, from three of the clock (rather six) till nine , putting forth a long
tail. Such also was that most resplendent comet described by Aristotle ,
lib. 1, Meteor. 6. The head whereo f could not be seen , because it had set
before the sun, or at least was hid under the surfs rays ; but next day
it was seen as well as might be ; for, having left the sun but a very lit¬
tle way, it set immediately after it. And the scattered light of the head ,
obscured by the too great splendour (of the tail) did not yet appear. But
afterwards (as Aristotle says) when the splendour (of the tail) was now
diminished (the head of), the comet recovered its native brightness ; and
the splendour (of its tail) reached now to a third part of the heavens (that
is, to 60°). This appearance was in the wilder season (an. 4, Olymp.
101), and, rising to Orion’s girdle , it there vanished away. It is true
that the comet of 1618, which came out directly from under the sun’s rays
with a very large tail, seemed to equal, if not to exceed, the stars of the
first magnitude; but, then, abundance of other comets have appeared yet
greater than this, that put forth shorter tails; some of which are said

OF NATURAL PHILOSOPHY.

495

Book III.]

to have appeared as big as Jupiter, others as big as Venus, or even as
the moon.

We have said, that comets are a sort of planets revolved in very eccen¬
tric orbits about the sun; and as, in the planets which are without tails,
those are commonly less which are revolved in lesser orbits, and nearer to
the sun, so in comets it is probable that those which in their perihelion ap¬
proach nearer to the sun ate generally of less magnitude, that they may
not agitate the sun too much by their attractions. But. as to the trans¬
verse diameters of their orbits, and the periodic times of their revolutions,
I leave them to be determined by comparing comets together which after
long intervals of time return again in the same orbit. In the mean time,
the following Proposition may give some light in that inquiry.

PROPOSITION XLII. PROBLEM XXII.

To corred a cometfs trajectory found as above.

Operation 1. Assume that position of the plane of the trajectory which
was determined according to the preceding proposition; and select three
places of the comet, deduced from very accurate observations, and at great
distances one from the other. Then suppose A to represent the time be¬
tween the first observation and the second, and B the time between the
second and the third; but it will be convenient that in one of those times
the comet be in its perigeon, or at least not far from it. From those ap¬
parent places find, by trigonometric operations, the three true places of the
comet in that assumed plane of the trajectory; then through the places
found, and about the centre of the sun as the focus, describe a conic section
by arithmetical operations, according to Prop. XXI., Book 1. Let the
areas of this figure which are terminated by radii drawm from the sun to
the places found be D and E; to wit, D the area between the first observa¬
tion and the second, and E the area between the second and third; and let
T represent the whole time in which the whole area D + E should be de¬
scribed with the velocity of the comet found by Prop. XVI., Book 1.

Oper. 2. Retaining the inclination of the plane of the trajectory to the
plane of the ecliptic, let the longitude of the nodes of the plane of the tra¬
jectory be increased by the addition of 20 or 30 minutes, which call P.
Then from the aforesaid three observed places of the comet let the three
true places be found (as before) in this new plane; as also the orbit passing
through those places, and the two areas of the same described between the
two observations, which call d and e ; and let t be the whole time in which
the whole area d + e should be described.

OrER. 3. Retaining the longitude of the nodes in the first operation, let
the inclination of the plane of the trajectory to the plane of the ecliptic be
increased by adding thereto 20' or 30', which call Q. Then from the

496

THE MATHEMATICAL PRINCIPLES

[Book 111

aforesaid three observed apparent places of the comet let the three true
places be found in this new plane, as well as the orbit passing through
them, and the two areas of the same described between the observation,
which call d and £; and let r be the whole time in which the whole area
6 4- £ should be described.

Then taking C to 1 as A to B ; and G to l^as D to E; and g to 1 as
d to e; and y to 1 as d to £; let S he the true time between the first ob¬
servation and the third ; and, observing well the signs -f- and —, let such
numbers m and n be found out as will make 2G — 2C, = mG — mg
4- wG — ny ; and 2T — 2S = mT — ml + //T — nr. And if, in
the first operation, I represents the inclination of the plane of the trajec¬
tory to the plane of the ecliptic, and K the longitude of either node, then
I + 7/Q, will be the true inclination of the plane of the trajectory to the
plane of the ecliptic, and K 4* mV the true longitude of the node. And.
lastly, if in the first, second, and third operations, the quantities R, r, and

p, represent the parameters of the trajectory, and the quantities 4-,-y,

the transverse diameters of the same, then R 4- mr — mK -f ??p — wR

will be the true parameter, and t— -:-:--—v- ■=- will be the

r L -f- ml — mL + n A —

true transverse diameter of the trajectory which the comet describes; and
from the transverse diameter given the periodic time of the comet is also
given. Q..E.I. But the periodic times of the revolutions of comets, and
the transverse diameters of their orbits, cannot be accurately enough de¬
termined but by comparing comets together which appear at different
times. If, after equal intervals of time, several comets are found to have
described the same orbit, we may thence conclude that they are all but one
and the same comet revolved in the same orbit; and then from the times
of their revolutions the transverse diameters of their orbits will be given,
and from those diameters the elliptic orbits themselves will be determined.

To this purpose the trajectories of many comets ought to be computed,
supposing those trajectories to be parabolic; for such trajectories will
always nearly agree with the phcenornena, as appears not only from the
parabolic trajectory of the comet of the year 1680, which I compared
above with the observations, but likewise from that of the notable comet
which appeared in the year 1664 and 1665, and was observed by Hevelius .
who, from his own observations, calculated the longitudes and latitudes
thereof, though with little accuracy. But from the same observations Dr.
Halley did again compute its places; and from those new places deter¬
mined its trajectory, finding its ascending node in n 21° 13' 55" ; the in-
clinationof the orbit to the plane of the ecliptic 21° 18 f 40" ; the dis¬
tance of its perihelion from the node, estimated in the comet’s orbit, 49°
27' 30",'its perihelion in Si 8° 40' 30", with heliocentric latitude south

Book III.] of natural philosophy. 497

16° UP 45" ; the comet to have been in its perihelion November 2i A . llu.
52' P.M. equal time at London , or 13 h . 8' at Duntzick , O. S.; and that
the latus rectum of the parabola was4102S6 such parts as the sun’s mean
distance from the earth is supposed to contain 100000. And how nearly
the places of the comet computed in this orbit agree with the observations,
will appear from the annexed table, calculated by Dr. Halley.

Appar.Time
at Uantzick.

The observed Distances of the Comet fiom

The observed Places.

1 lie Places
computed in
the orb.

December
d. h. '
3.18.29^

O t II

The Lion’s heart 46.24.20

The Virgin’s spike 22.52.10

O / ff

Long. ^ 7.01.00

Lat. S. 21.39. 0

o / 1.

=2= 7. 1.29
21.38.50

4.18. li

The Lion’s heart 46. 2.45

The Virgin’s spike 23.52.40

Long. 6.15. 0

Lat. S. 22.24. 0

=£= 6.16. 5

22.24. 0

7.17.48

The Lion’s heart 44.48. 0

The Virgin’s spike 27.56.40

Long. =£= 3. 6. 0

Lat. S. 25.22. 0

3W 3. 7.33
25.21.40

17.14.43

The Lion’s heart 53.15.15

Orion’s right shoulder 45.43.30

Long. 0. 2.56. 0
Lat. S. 49.25. 0

U 2.56. C
49.25. 0

19. 9.25

1

Proeyon 35.13.50

Bright star of Whale’s jaw 52 56. 0

Long. 11 28 40.30
Lat. S. 45.48. 0

n 28.43. 0
45 46. 0

20. 9.53 h

Procyon 40.49. 0

Bright star of Whale’s jaw 40.04. 0

Long. 11 13.03. 0
Lat. S. 39.54. 0

n 13. 5. 0
39.53. 0

21. 9. 9 h

Orion’s right shoulder 26.21.25

Bright star of Whale’s jaw 29.28. 0

Long, n 2.16. 0
Lat. S, 33.41. 0

n 2 18 30
33.39.40

22. 9. 0

Orion’s right shoulder 29.47. 0

Bright starof Whale’s jaw 20.29.30

Long. b 24.24. 0
Lat. S. 27.45. 0

b 24.27. 0
27.46. 0

26. 7.58

The bright star of Aries 23.20. 0
Aldebaran 26.44. 0

Long. b 9. 0. 0
Lat. S. 12.36. 0

b 9. 2.28
12.34.13

27. 6.45

The bright star of Aries 20.45. 0
Aldebaran 28.10. 0

b 7. 8.45
10.23.13

128. 7.39

1

The bright star of Aries 18.29. 0
Palilicium 29.37. 0

Long, b 5.24.45
Lat.^S. 8.22.50

b 5 27.5:.
8.23 37

31. 6.45

Andromeda’s girdle 30.48.10

Palilicium 32.53.30

Long. b 2. 7.40
Lat. S. 4.13. 0

b 2. 8.2(
4.16.25

1 Jan. 1665
| 7. 7.37 h

Andromeda’s girdle 25.11. 0

Palilicium 37.12.25

Long. T 28.24.47
Lat. N. 0.54. 0

T 28.24. C
0.53. (

13. 7. 0

Andromeda’s head 28. 7.10

Palilicium 38.55.20

Long. T 27. 6.54
Lat. N. 3. 6.50

T 27. 6.39
3. 7.4u

24. 7.29

Andromeda’s girdle 20.32.15

Palilicium 40. 5. 0

Long. 26.29.15

Lat. N. 5.25.50

T 26.28.50
5.26. C

Feb.

7. 8.37

Long. <r> 27. 4.46
Lat. N. 7. 3.29

T 27.24.55
7. 3.15

*22. 8.46

1

*

Long. 28.29.46

Lat. N. 8.12.36

T 28.29.58
8.10.25!

1 March

j 1. 8.16

Long. T 29.18 15
Lat. N. 8.36.26

T 29.18.20l

8.36.12'

7 . 8.37

Long, b 0. 2.48
Lat. N. 8.56.30

b 0. 2.42,
8.56.56/

In February , the beginning of the year 1665, the first star of Aries,
which I shall hereafter call y, was in hp 2S° 30' 15", with 7° S' 58" north

32

498 the mathematical principles [Book III.

Iat.; the second star of Aries was in HP 29° 17' IS ', with 8° 28' 16" north
lat.; and another star of the seventh magnitude, which I call A, was in
i r> 28° 24' 45", with 8° 2S' 33" north lat. The comet Feb. 7< ] . 7 h . 30' at
Paris (that is, Feb. 7'K S h . 3 7' at Dantzick) O. S. made a triangle with
those stars y and A, which was right-angled in y; and the distance of the
comet from the star y was equal to the distance of the stars y and A, that
is, 1° 19' 46" of a great circle; and therefore in the parallel of the lati¬
tude of the star y it was 1° 20' 26". Therefore if from the longitude of
the star y there be subducted the longitude 1° 20' 26", there will remain
the longitude of the comet HP 27° 9' 49". M. Auzont, from this observa¬
tion of his, placed the comet in HP 2 7° O', nearly ; and, by the scheme in
which Dr. Hooke delineated its motion, it was then in HP 26 3 59' 24 '. 1

place it in HP 27° 4' 46", taking the middle between the two extremes.

From the same observations, M. Anzovt made the latitude of the comet
at that time 7° and 4' or 5' to the north ; but he had done better to have
made it 7° 3' 29", the difference of the latitudes of the comet and the star
y being equal to the difference of the longitude of the stars y and A.

February 22''. 7 h . 30' at London , that is, February 22 d . 8 h . 46' at
Dantzick , the distance of the comet from the star A, according to Dr.
Hooke's observation, as was delineated by himself in a scheme, and also
by the observations of M. Anzovt , delineated in like manner by M. Petit ,
was a fifth part of the distance between the star A and the first star of
Aries, or 15' 57" ; and the distance of the comet from a right line joining
the star A and the first of Aries was a fourth part of the same fifth part,
that is, 4'; and therefore the comet was in HP 2S° 29' 46", with 8° 12'
36" north lat.

March 1, 7 h . 0' at London, that is, March 1, S h . 16' at Dantzick, the
comet was observed near the second star in Aries, the distance between
them being to the distance between the first and second stars in Aries, that
is, to 1° 33', as 4 to 45 according to Dr. Hooke , or as 2 to 23 according
to M. Gottignies. And, therefore, the distance of the comet from the
second star in Aries was S' 16" according to Dr. Hooke, or 8' 5" according
to M. Gottignies ; or, taking a mean between both, S' 10". But, accord¬
ing to M. Gottignies, the comet had gone beyond the second star of Aries
about a fourth or a fifth part of the space that it commonly went over in
a day, to wit, about 1' 35" (in which he agrees very well with M. Auzont );
or, according to Dr. Hooke, not quite so much, as perhaps only 1'. Where-
fore if to the longitude of the first star in Aries we add 1', and 8' 10" to
its latitude, we shall have the longitude of the comet HP 29° IS', with 8°
36' 26" north lat.

March 7, 7 h . 30' at Paris (that is, March 7, 8 h . 37' at Dantzick),
from the observations of M. Auzout, the distance of the comet from the
second star in Aries was equal to the distance of that star from the star

OF NATURAL PHILOSOPHY.

499

Book III.]

A, that is, 52/29" ; and the difference of the longitude of the comet and
the second star in Aries vras 45' or 46', or, taking a mean quantity, 45'
30" ; and therefore the comet was in 3 0° 2' 48". From the scheme of
the observations of M. Anzuut, constructed by M. Petit , Hevelius collected
the latitude of the comet 8 3 54'. But the engraver did not rightly trace
the curvature of the comets way towards the end of the motion; and
Hevelius , in the scheme of M. AuzouVs observations which he constructed
himself, corrected this irregular curvature, and so made the latitude of the
comet S° 55' 30". And, by farther correcting this irregularity, the lati¬
tude may become 8° 56', or S° 57'.

This comet was also seen March 9, and at that time its place must have
been in 3 0° IS', with 9° 3^' north lat. nearly.

This comet appeared three months together, in which space of time it
travelled over almost six signs, and in one of the days thereof described
almost 20 deg. Its course did very much deviate from a great circle, bend¬
ing towatds the north, and its motion towards the end from retrograde be¬
came direct; and, notwithstanding its course was so uncommon, yet by the
table it appears that the theory, from beginning to end, agrees with the
observations no less accurately than the theories of the planets usually do
with the observations of them ; but we are to subduct about 2' when the
comet was swiftest, which we may effect by taking off 12" from the angle
between the ascending node and the perihelion, or by making that angle
49° 27' 18". The annual parallax of both these comets (this and the
preceding) was very conspicuous, and by its quantity demonstrates the an¬
nual motion of the earth in the orbis mag tins.

This theory is likewise confirmed by the motion of that comet, which
in the year 1683 appeared retrograde, in an orbit whose plane contained
almost a right angle with the plane of the ecliptic, and whose ascending
node (by the computation of Dr. Halley) was in ng 23° 23'; the inclina¬
tion of its orbit to the ecliptic 83° IT ; its perihelion in n 25° 29' 30"
its perihelion distance from the sun 56020 of such parts as the radius of
the orbis mag mis contains 100000; and the time of its perihelion July
2 (l . 3 h . 50'. And the places thereof, computed by Dr. Halley in this orbit,
are compared with the places of the same observed by Mr. Flamsted. in
the following table:—

500

THE MATHEMATICAL PRINCIPLES

[Book III.

Sun’s place

Cornel’s
Lorur. com.

Lat.Nor.

lompnt.

Comet’s
Long. obs’d

Lat.Nor.

observ’d

Diff.

Long.

Diff.

Lat.

O » II

u / ;/

c > n

0111

O II

i n

i it

July 13.12.55

£1 1.02.30

25 13.05.42

29.28.13

25 13. 6.42

29.28.20

+ 1.00

+ 0.07

15.11.15

2.53.12

11.37.48

29 34. 0

11.39.43

29.34.50

+ 1.55

+ 0.50

17.10.20

4.45.45

10. 7. 6

29.33.30

10. 8.40

29.34. 0

+ 1.34

+ 0.30

23.13 40

10.38.21

5.10.27

28.51.42

5.11.30

28.50.28

+ 1.03

— 1.14j

25.14. 5

12.35.28

3.27.53

24.24.47

3.27. 0

28.23.40

— 0.53

— i. 7

31. 9.42

18.09.22

n 27.55. 3

26.22-52

U 27.54.24

26.22.25

— 0.39

— 0.27|

31.14.55

18.21.53

27.41. 7

26.16.57

27.41. 8

26.14.50

+ 0. 1

— 2. 71

Aug . 2.14 56

20.17.16

25.29.32

25.16.19

25.28.46

25.17.28

— 0.46

+ 1. 9

4.10.49

22 02 50

23.18.20

24.10.49

23.16.55

24.12.19

— 1.25

+ 1.30

6.10 9

23.56.45

20.42.23

22.17. 5

20.40.32

22.49. 5

— 1 51

+ 2. 0

9.10.26

26.50.52

16 7.57

20. 6.37

16. 5.55

20. 6.10

— 2 . 2

— 0.27

15.14. 1

W 2.47.13

3.30.48

11.37.33

3.26.18

11.32. 1

— 4.30

— 5.32

16.15.10

3.48. 2

0.43. 7

9.34.16

0.41.55

9.34.13

— 1.12

— 0. 3

18.15.44

5.45.33

» 24.52.53

5.11.15

S 24.49. 5

5. 9.11

— 3.48

— 2. 4

South.

South

1

22.14.44

9.35.49

11. 7.14

5.16.58

11.07.12

5.16.58

— 0. 2

— 0. 3

23 15.52

10.36.48

7. 2.18

8.17. 9

7. 1.17

8.16.41 j

— 1. 1

— 0.28

26.16. 2

13.31.10

°P 24.45.31

16.38. 0

|T 24.44.001

16.38.201

— 1.31

+ 0.20

This theory is yet farther confirmed by the motion of that retrograde
comet which appeared in the year 1682. The ascending node of this (by
Dr. Halley’s computation) was in S 21° 16'30"; the inclination of its
orbit to the plane of the ecliptic 17° 56' 00" ; its perihelion in ~ 2° 52'
50" ; its perihelion distance from the sun 5S32S parts, of which the radius
of the or bin magnus contains 100000; the equal time of' the comet’s
being in its perihelion Sept. 4 d . 7 h . 39'. And its places, collected from
Mr. Flamsted’s observations, are compared with its places computed from
our theory in the following table :—

1682

App. Time.

Sun’s place

Comet’s
Lon. comp.

Lat.Nor

comp.

Com. Long,
observed.

Lat Nor
observ.

Diff.

Long.

Diff.

Lat.

a. h. '

V 1 II

i a

O 1 II

9 1 II \

O 1 II

i n

i n

Avg 19.16.38

W 7. 0. 7

il 18.14 28

25.50. 7

|£1 18.14.40

25.49.55

— 0.12

+ 0.12

20.15.38

7.55.52

24.46.23

26.14.42

24.46.22

26.12.52

+ 0. 1

+ 1.50

21. 8.21

8.36 14

29.37.15

26.20. 3

29.38.02

26.17.37

— 0.47

+ 2.26

22. 8. 8

9.33.55

HJZ 6.29.53

26. 8.42

w

6.30. 3

26. 7.12

— 0.10

q_ 1.30

29.08.20

16 22.40

- 12.37.54

18.37.47

12.37.49

18.34. 5

+ 0. 5

-{- 3 42

30. 7.45

17.19.41

15.36. 1

17.26.43

15.35.18

17.27.17

i- 0.43

— 0.34

Sept. 1. 7.33

19.16. 9

20.30.53

15.13. 0

20.27. 4

15. 9.49

+ 3.49

+ 3.11

4. 7.22

22.11.28!

25.42. 0

12.23.48

25.40 58

12.22. 0

+ 1. 2

4- 1.48

5. 7.32

23 10.291

27. 0.46

11.33.08

26.59.24

11.33.51

+ 1.22

— 0.43

8. 7.16

26. 5.58|

29.58.44

9.26.46

29.58.45

9.26.43.

— 0 .. 1

+ 0. 3

9. 7.261

27. 5. 9

0.44 10

8.49.10

0 44. 4

8.48.25

-f- 0. 6

4- 0.45

This theory is also confirmed by the retrograde motion of the comet that
appeared in the year 1723. The ascending node of this comet (according
to the computation of Mr. Bradley , Savilian Professor of Astronomy at
Oxford) was in T 14° 16'. The inclination of the orbit to the plane of
the ecliptic 49° 59'. Its perihelion was in 3 12° 15' 20". Its perihelion
distance from the sun 998651 parts, of which the radius of the orbis mag•
mts contains 1000000, and the equal time of its perihelion September 16 d

OF NATURAL PHILOSOPHY.

501

Book III.]

16 h . 10'. The places of this comet computed in this orbit by Mr. Bradley ,
and compared with the places observed by himself, his uncle Mr. Pound,
and Dr. Halley , may be seen in the following table.

r

1723

Eq. time.

Cornet’s
Lon^ obs.

Lat.Nor

obs.

Comet’s
Lon. com.

Lat.Nor

comp.

Diff.

Lon.

Diff.

Lat.

d. h. '

O i II

w / n

o t II

u i a

V

n

Of. 9.8. 5

~ 7.22.15

5. 2. 0

~ 7.21.26

5. 2.47

+ 49

— 47

10.6.21

6.41.12

7.44.13

6.41.42

7.43.18

— 50

+ 55

12.7.22

5 39.58

11.55. 0

5.40.19

11 54 55

— 21

+ 5

14.8.57

4.59.49

14.43.50

5. 0.37

14.44. 1

— 48

_ 11

15.6.35

4.47.41

15.40.51

4.47.45

15.40.55

— 4

_ 4

21.f 22

4. 2.32

19.41.49

4. 2.21

19.42. 3

+ H

-14

22.6.24

3.59. 2

20. 8.12

3 59.10

20. 8.17

- 8

— 5

24.8. 2

3.55.29

20.55.18

3 55.11

20.55. 9

+ 18

+ 9

29.8.56

3.56.17

22.20.27

3.56.42

22.20.10

— 25

+ 17

30.6.20

3.58. 9

22.32.2$

3.58.17

22.32.12

— 8

+ 16

Nov. 5.5.53

4.16.30

23.38 33

4.16.23

23.38. 7

+ 7

+ 26

8.7. 6

4.29.36

24. 4.30

4.29.54

24. 4.40

— 18

— 10

14.6 20

5. 2.16

24.48.46

5. 2.51

24.48 16

— 35

+ 30

20.7.45

5.42.20

25.24.45

5.43.13

25.25.17

— 53

— 32

Dec. 7.6.45

8. 4.13

26.54.18

8. 3.55

26.53.42

+ 18

+ 36

From these examples it is abundantly evident that the motions of com¬
ets are no less accurately represented by our theory than the motions of the
planets commonly are by the theories of them ; and, therefore, by means of
this theory, we may enumerate the orbits of comets, and so discover the
periodic time of a comet’s revolution in any orbit; whence, at last, wc
shall have the transverse diameters of their elliptic orbits and their aphe¬
lion distances.

That retrograde comet which appeared in the year 1607 described an
orbit whose ascending node (according to Dr. Halley's computation) was in
b 20° 21'; and the inclination of the plane of the orbit to the plane of
the ecliptic 17° 2';. whose perihelion was in ca 2° 16'; and its perihelion
distance from the sun 5S680 of such parts as the radius of the orbis mag-
nus contains 100000; and the comet was in its perihelion October 16 1 . 3'\
50'; which orbit agrees very nearly with the orbit of the comet which was
seen in 1682. If these were not two different comets, but one and the
same, that comet will finish one revolution in the space of 75 years; and
the greater axis of its orbit will be to the greater axis of the orbis magnus
as v/ 3 :75 X 75 to 1, or as 1778 to 100, nearly. And the aphelion dis¬
tance of this comet from the sun will be to the mean distance of the earth
from the sun as about 35 to 1; from which data it will be no hard matter
to determine the elliptic orbit of this comet. But these things are to be
supposed on condition, that, after the space of 75 years, the same comet
shall return again in the same orbit. The other comets seem to ascend to
greater heights, and to require a longer time to perform their revolutions.

But, because of the great number of comets, of the great distance of their

502

THE MATHEMATICAL PRINCIPLES

[Book 111

aphelions from the sun, and of the slowness of their motions in the aphe¬
lions, they will, by their mutual gravitations, disturb each other; so that
their eccentricities and the times of their revolutions will be sometimes a
little increased, and sometimes diminished. Therefore we are not to ex¬
pect that the same comet will return exactly in the same orbit, and in the
same periodic times: it will be sufficient if we find the changes no greater
than may arise from the causes just spoken of.

And hence a reason may be assigned why comets are not comprehend*ed
within the limits of a zodiac, as the planets are; but, being confined to no
bounds, are with various motions dispersed all over the heavens; namely,
to this purpose, that in their aphelions, where their motions are exceedingly
slow, receding to greater distances one from another, they may suffer less
disturbance from their mutual gravitations: and hence it is that the comets
which descend the lowest, and therefore move the slowest in their aphelions,
ought also to ascend the highest.

The comet which appeared in the year 1680 was in its perihelion less
distant from the sun than by a sixth part of the sun’s diameter; and be¬
cause of its extreme velocity in that proximity to the sun, and some density
of the sun’s atmosphere, it must have suffered some resistance and retarda¬
tion; and therefore, being attracted something nearer to the sun in evu-y
revolution, will at last fall down upon the body of the sun. Nay, in its
aphelion, where it moves the slowest, it may sometimes happen to be yet
farther retarded by the attractions of other comets, and in consequence of
this retardation descend to the sun. So fixed stars, that have been gradu¬
ally wasted by the light and vapours emitted from them for a long time,
may be recruited by comets that fall upon them; and from tlrs fresh sup¬
ply of new fuel those old stars, acquiring new splendor, may pass for new
stars. Of this kind are such fixed stars as appear on a sudden, and shine
with a wonderful brightness at first, and afterwards vanish by little and
little. Such was that star which appeared in Cassiopeia 1 s chair; which
Cornelius Gemma did not see upon the Sth of November , 1572, though
he was observing that part of the heavens upon that very night, and the
sky was perfectly serene; but the next night [November 9) he saw it
shining much brighter than any of the fixed stars, and scarcely inferior to
Venus in splendor. Tycho Brahe saw it upon the 11th of the same month,
when it shone with the greatest lustre; and from that time he observed it
to decay by little and little; and in 16 months’ time it entirely disap¬
peared. In the month of November , when it first appeared, its light was
equal to that of Venus. In the month of December its light was a little
diminished, and was now become equal to that of Jupiter. In January
1573 it was less than Jupiter , and greater than Sirius; and about the
end of February and the beginning of March became equal to that star.
In the months of April and May it was equal to a star of the second mag-

OF NATURAL PHILOSOPHY.

503

Lioox 111.]

uitude; in June , July , and August, to a star of the third magnitude; in
September , October, and November , to those of the fourth magnitude; in
December and January 1574 to those of the fifth; in February to those
of the sixth magnitude; and in March it entirely vanished. Its colour at
the beginning was clear, bright, and inclining to white; afterwards it
turned a little yellow; and in March 1573 it became ruddy, like Mars or
Aldebarau: in May it turned to a kind of dusky whiteness, like that we
observe in Saturn ; and that colour it retained ever after, but growing al¬
ways more and more obscure. Such also was the star in the right foot ol
Serpentarins, which Kepler s scholars first observed September 30, O.S.
1604, with a light exceeding that of Jupiter , though the night before it
was not to be seen; and from that time it decreased by little and little,
and in 15 or 16 months entirely disappeared. Such a new star appearing
with an unusual splendor is said to have moved Hipparchus to observe,
and make a catalogue of, the fixed stars. As to those fixed stars that ap¬
pear and disappear by turns, and increase slowly and by degrees, and
scarcely ever exceed the stars of the third magnitude, they seem to be of
another kind, which revolve about their axes, and, having a light and a
dark side, shew those two different sides by turns. The vapours which
arise from the sun, the fixed stars, and the tails of the comets, may meet
at last with, and fall into, the atmospheres of the planets by their gravity,
and there be condensed and turned into water and humid spirits; and from
thence, by a slow heat, pass gradually into the form of salts, and sulphurs,
and tinctures, and mud, and clay, and sand, and stones, and coral, and other
terrestrial substances.

GENERAL SCHOLIUM.

The hypothesis of vortices is pressed with many difficulties. That every
planet by a radius drawn to the sun may describe areas proportional to the
times of description, the periodic times of the several parts of the vortices
should observe the duplicate proportion of their distances from the sun;
but that the periodic times of the planets may obtain the sesquiplicate pro¬
portion of their distances from the sun, the periodic times of the parts of
the vortex ought to be in the sesquiplicate proportion of their distances.
That the smaller vortices may maintain their lesser revolutions about
Saturn , Jupiter , and other planets, and swim quietly and undisturbed in
the greater vortex of the sun, the periodic times of the parts of the sun’s
vortex should be equal; but the rotation of the sun and planets about their
axes, which ought to correspond with the motions of their vortices, recede
far from all these proportions. The motions of the comets are exceedingly
regular, are governed by the same laws with the motions of the planets,
and can by no means be accounted for by the hypothesis of vortices ; for
comets are carried with very eccentric motions through all parts of the

THE MATHEMATICAL PRINCIPLES

501

[Book IIL

heavens indifferently, with a freedom that is incompatible with the notion
of a vortex.

Bodies projected in our air suffer no resistance but from the air. With¬
draw the air, as is done in Mr. Boyle’s vacuum, and the resistance ceases;
for in this void a bit of line down and a piece of solid gold descend with
equal velocity. A,nd the parity of reason must take place in the celestial
spaces above the earth’s atmosphere; in which spaces, where there is no
air to resist their motions, all bodies will move with the greatest freedom;
and the planets and comets will constantly pursue their revolutions in or¬
bits given in kind and position, according to the laws above explained; but
though these bodies may, indeed, persevere in their orbits by the mere laws
of gravity, yet they could by no means have at first derived the regular
position of the orbits themselves from those laws.

The six primary planets are revolved about the sun in circles concentric
with the sun, and with motions directed towards the same parts, and al¬
most in the same plane. Ten moons are revolved about the earth, Jupiter
and Saturn, in circles concentric with them, wi'.h the same direction of
motion, and nearly in the planes of the orbits of those planets; but it is
not to be conceived that mere mechanical causes could give birth to so
many regular motions, since the comets range over all parts of the heavens
in very eccentric orbits; for by that kind of motion they pass easily through
the orbs of the planets, and with great rapidity; and in their aphelions,
where they move the slowest, and are detained the longest, they recede to
the greatest distances from each other, and thence suffer the least disturb¬
ance from their mutual attractions. This most beautiful system of the sun,
planets, and comets, could only proceed from the counsel and dominion of an
intelligent and powerful Being. And if the fixed stars are the centres of oth¬
er like systems, these, being formed by the like wise counsel, must be all sub¬
ject to the dominion of One; especially since the light of the fixed stars is
of the same nature with the light of the sun, and from every system light
passes into all the other systems: and lest the systems of the fixed stars
should, by their gravity, fall on each other mutually, he hath placed those
systems at immense distances one from another.

This Being governs all things, not as the soul of the world, but as Lord
over all; and on account of his dominion he is wont to be called Lord God
7Ta*TOKpdTG)p } or Universal Ruler ; for God is a relative word, and has a
respect to servants; and Deity is the dominion of God not over his own
body, as those imagine who fancy God to be the soul of the world, but over
servants. The Supreme God is a Being eternal, infinite, absolutely per¬
fect ; but a being, however perfect, without dominion, cannot be said to be
Lord God; for we say, my God, your God, the God of Israel, the God of
Gods, and Lord of Lords; but we do not say, my Eternal, your Eternal,
the Eternal of Israel , the Eternal of Gods; we do not say, my Infinite, oi

Hook III.J

Of* NATURAL PHILOSOPHY.

505

my Perfect: these are titles which have no respect to servants. The word
God* usually signifies Lord; but every lord is not a God. It is the do¬
minion of a spiritual being which constitutes a God: a true, supreme, or
imaginary dominion makes a true, supreme, or imaginary God And from
his true dominion it follows that the true God is a living, intelligent, and
powerful Being; and, from his other perfections, that he is supreme, or
most perfect. He is eternal and infinite, omnipotent and omniscient; that
is, his duration reaches from eternity to eternity.; his presence from infinity
to infinity; he governs all things, and knows all things that are or can be
done. He is not eternity or infinity, but eternal and infinite; he is not
duration or space, but he endures and is present. He endures for ever, and
is every where present; and by existing always and every where, he consti¬
tutes duration and space. Since every particle of space is always , and
every indivisible moment of duration is every where , certainly the Maker
and Lord of all things cannot be never and no where. Every soul that
has perception is, though in different times and in different organs of sense
and motion, still the same indivisible person. There are given successive
parts in duration, co-existent parts in space, but neither the one nor the
other in the person of a man, or his thinking principle; and much less
can they be found in the thinking substance of God. Every man, so far
as he is a thing that has perception, is one and the same man during his
whole life, in all and each of his organs of sense. God is the same God,
always and every where. He is omnipresent not virtually only, but also
substantially ; for virtue cannot subsist without substance. In himf are
all things contained and moved; yet neither affects the other: God suffers
nothing from the motion of bodies; bodies find no resistance from the om¬
nipresence of God. It is allowed by all that the Supreme God exists
necessarily; and by the same necessity he exists always and every where.
Whence also he is all similar, all eye, all ear, all brain, all arm, all power
to perceive, to understand, and to act; but in a manner not at all human,
in a manner not at all corporeal, in a manner utterly unknown to us. As
a blind man has no idea of colours, so have we no idea of the manner by

* Dr. Pocock derives the Latin word Deus from the Arabic du (in the oblique case di).
which signifies Lord. And in this sense princes are called gods, Psal. Ixxxii. ver. 6; and
John x. ver. 35. And Closes is called a god to his brother Aaron, and a god to rharaoh
(Exod . iv. ver. 16; and vii. ver. 1). And in the same sense the souls of dead princes were
formerly, by the Heathens, called gods, but falsely, because of their want of dominion.

t This was the opinion of the Ancients. So Pythagoras, in Cicer. de Nat. Deor. lib. i
Thales, Anaxagoras, Virgil, Georg, lib. iv. ver. 220; and jEneid, lib. vi. ver. 721. Philo
Allegor, at the beginning of lib. i. Aratus, in his Phsenom. at the beginning. So also the
sacred writers; as St. Paul, Acts, xvii. ver 27,28. St. John's Gosp. chap. xiv. ver. 2. Mo¬
ses, in Deut. iv. ver. 39; and x ver. 14. David, Psal. cxxxix. ver. 7, 8, 9. Solomon, 1
Kings, viii. ver. 27. Job, xxii. ver. 12, 13, 14. Jeremiah, xxiii. ver. 23, 24. The Idolaters
supposed the sun. moon, and stars, the souls of men, and other parts of the world, to be
parts of the Supreme God, and therefore to be worshipped; but erroneously.

506

THE MATHEMATICAL PRINCIPLES

[Book Ill.

which the all-wise God perceives and understands all things. He is ut¬
terly void of all body and bodily figure, and can therefore neither he seen,
nor heard, nor touched; nor ought he to be worshipped under the repre¬
sentation of any corporeal thing. We have ideas of his attributes, but
what the real substance of any thing is we know not. In bodies, we see
only their figures and colours, we hear only the sounds, we touch only their
outward surfaces, we smell only the smells, and taste the savours; but their
inward substances are not to be known either by our senses, or by any
reflex act of our minds: much less, then, have we any idea of the sub¬
stance of God. We know him only by his most wise and excellent con¬
trivances of things, and final causes: we admire him for his perfections;
but we reverence and adore him on account of his dominion: for we adore
him as his servants; and a god without dominion, providence, and final
causes, is nothing else but Fate and Nature. Blind metaphysical neces¬
sity, which is certainly the same always and every where, could produce
no variety of things. All that diversity of natural things which we find
suited to different times and places could arise from nothing but the ideas
and will of a Being necessarily existing. But, by way of allegory, God
is said to see, to speak, to laugh, to love, to hate, to desire, to give, to re¬
ceive, to rejoice, to be angry, to fight, to frame, to work, to build; for all
our notions of God are taken from the ways of mankind by a certain
similitude, which, though not perfect, has some likeness, however. And
thus much concerning God ; to discourse of whom from the appearances
of things, does certainly belong to Natural Philosophy.

Hitherto we have explained the phenomena of the heavens and of our
sea by the power of gravity, but have not yet assigned the cause of this
power. This is certain, that it must proceed from a cause that penetrates
3 to the very centres of the sun and planets, without suffering the least
diminution of its force; that operates not according to the quantity of
the surfaces of the particles upon which it acts (as mechanical causes use
to do), but according to the quantity of the solid matter which they con¬
tain. and propagates its virtue on all sides to immense distances, decreasing
always in the duplicate proportion of the distances. Gravitation towards
the sun is made up out of the gravitations towards the several particles
of which the body of the sun is composed ; and in receding from the sun
decreases accurately in the duplicate proportion of the distances as far as
the orb of Saturn, as evidently appears from the quiescence of the aphe¬
lions of the planets ; nay, and even to the remotest aphelions of the comets,
if those aphelions are also quiescent. But hitherto I have not been able
to discover the cause of those properties of gravity from phenomena, and
I frame no hypotheses; for whatever is not deduced from the phenomena
is to be called an hypothesis; and hypotheses, whether metaphysical or
physical, whether of occult qualities or mechanical, have no place in ex

Book III.]

OF NATURAL PHILOSOPHY.

507

perimental philosophy. In this philosophy particular propositions are
inferred from the phenomena, and afterwards rendered general by induc¬
tion. Thus it was that the impenetrability, the mobility, and the impul¬
sive force of bodies, and the laws of motion and of gravitation, were
discovered. And to us it is enough that gravity does really exist, and act
according to the laws which we have explained, and abundantly serves to
account for all the motions of the celestial bodies, and of our sea.

And now we might add something concerning a certain most subtle
Spirit which pervades and lies hid in all gross bodies; by the force and
action of which Spirit the particles of bodies mutually attract one another
at near distances, and cohere, if contiguous; and electric bodies operate to
greater distances, as well repelling as attracting the neighbouring corpus¬
cles ; and light is emitted, reflected, refracted, inflected, and heats bodies ;
and all sensation is excited, and the members of animal bodies move at the
comman d,of the_ will, namely, by the vibrations of this Spirit, mutually
propagated along the solid filaments of the nerves, from the outward or¬
gans of sense to the brain, and from the brain into the muscles. But these
are things that cannot be explained in few words, nor are we furnished
with that sufficiency of experiments which is required to an accurate deter¬
mination and demonstration of the laws by which this electric and elastic
Spirit operates.

END OF THE MATHEMATICAL PRINCIPLES.

THE

SYSTEM OF THE WOULD.

it, was the ancient opinion of not a few, in the earliest ages of philoso¬
phy. that the fixed stars stood immoveable in the highest parts of the world ;
that under the fixed stars the planets were carried about the sun; that the
earth, as one of the planets, described an annual course about the sun, while
by a diurnal motion it was in the mean time revolved about, its own axis;
and that the sun, as the common fire which served to warm the whole, was
fixed in the centre of the universe.

This was the philosophy taught of old by Philolaus , Aristarchus of
Samos, Plato in his riper years, and the whole sect of the Pythagoreans ;
and this was the judgment of Anaximander , more ancient than any of
them ; and of that wise Ling of the Ro?nans , Numa Pompilius, who, as
a symbol of the figure of the world with the sun in the centre, erected a
temple in honour of Vesta, of s xound form, and ordained perpetual fire to
be kept in the middle of it.

The Egyptians were early observers of the heavens; and from them,
probably, this philosophy was spread abroad among other nations; for from
them it was, and the nations about them, that the Greeks, a people of
themselves more addicted to the study of philology than of nature, derived
their first, as well as soundest, notions of philosophy; and in the vestal
ceremonies we may yet trace the ancient spirit of the Egyptians ; for it
was their way to deliver their mysteries, that is, their philosophy of things
above the vulgar way of thinking, under the veil of religious rites and
hieroglyphic symbols.

It is not to be denied but that Anaxagoras, Democritus, and others,
did now' and then start up, who would have it that the earth possessed the
centre of the world, and that the stars of all sort3 were revolved towards
the west about the earth quiescent in tk^ centre, some at a sw'ifter, others
at a slower rate.

H-owever, it w r as agreed on both sides that the motions of the celestial
bodies were performed in spaces altogether free ana void of resistance. The
w'him of solid orbs was of a later date, introduced by Eudoxus , Calippus ,
and Aristotle; when the ancient philosophy began to decline, and to give
nlace to the new prevailing fictions of the Greeks.

But. above all things, the phenomena of comets can by no means consist

512

THE SYSTEM OF THE WORLD.

with the notion of solid orbs. The Chaldeans , the mo3t learned astrono¬
mers of their time, looked upon the comets (which of ancient times before
had been numbered among the celestial bodies) as a particular sort of plan¬
ets, which, describing very eccentric orbits, presented themselves to our view
only by turns, viz., once in a revolution, when they descended into the
lower parts of their orbits.

And as it was the unavoidable consequence of the hypothesis of solid
orbs, while it prevailed, that the comets should be thrust down below the
moon, so no sooner had the late observations of astronomers restored the
comets to their ancient places in the higher heavens, but these celestial spaces
were at once cleared of the incumbrance of solid orbs, which by these ob¬
servations were broke into pieces, and discarded for ever.

Whence it was that the planets came to be retained within any certain
bounds in these free spaces, and to be drawn off from the rectilinear courses,
which, left to themselves, they should have pursued, into regular revolu¬
tions in curvilinear orbits, are questions which we do not know how the
ancients explained; and probably it was to give some sort of satisfaction
to this difficulty that solid orbs were introduced.

The later philosophers pretend to account for it either by the action of
certain vortices, as Kepler and Des Cartes ; or by some other principle of
impulse or attraction, as Borelli , Hooke , and others of our nation; for,
from the laws of motion, it is most certain that these effects must proceed
from the action of some force or other.

But our purpose is only to trace out the quantity and properties of this
force from the phenomena (p. 218), and to apply what we discover in some
simple cases as principles, by which, in a mathematical way, we may esti¬
mate the effects thereof in more involved cases ; for it would be endless and
impossible to bring every particular to direct and immediate observation.

We said, in a mathematical wax /, to avoid all questions about the na¬
ture or quality of this force, which we would not be understood to deter¬
mine by any hypothesis; and therefore call it by the general name of a
centripetal force, as it is a force which is directed towards some centre;
and as it regards more particularly a body in that centre, we call it circum¬
solar, circum-terrestrial, circum-jovial; and in like manner in respect of
other central bodies.

That by means of centripetal forces the planets may be retained in cer¬
tain orbits, we may easily understand, if we consider the motions of pro¬
jectiles (p. 75, 76, 77); for a stone projected is by the pressure of its own
weight forced out of the rectilinear path, which by the projection alone it
should have pursued, and made to describe a curve line in the air; and
through that crooked way is at last brought down to the ground; and the
greater the velocity is with which it is projected, the farther it goes before
it falls to the earth. We may therefore suppose the velocity to be so in

THE SYSTEM OF THE WORLD.

513

creased, that it would describe an arc of 1, 2, 5,10, 100, 1000 miles before
it arrived at the earth, till at last, exceeding the limits of the earth, it
should pass quite by without touching it.

Let AFB represent the surface of the earth, C its centre, YD, VE, VF,
the curve lines which a body would describe, if projected in an horizontal
direction from the top of an high mountain successively with more and

more velocity (p. 400); and, because the celestial motions are scarcely re¬
tarded by the little or no resistance of the spaces in which they are per¬
formed, to keep up the parity of cases, let us suppose either that there is
no air about the earth, or at least that it is endowed with little or no power
of resisting; and for the same reason tl at the body projected with a less
velocity describes the lesser arc YD, and with a greater velocity the greater
arc YE. and, augmenting the velocity, it goes farther and farther to F and
G, if the velocity was still more and more augmented, it would reach at
last quite beyond the circumference of the earth, and return to the moun¬
tain from which it was projected.

And since the areas which by this motion it describes by a radius drawn
to the centre of the earth are (by Prop. 1, Book 1, Princip. Math .) propor¬
tional to the times in which they are described, its velocity, when it returns
*o the mountain, will be no less than it was at first; and, retaining the
same velocity, it will describe the same curve over and over, by the same law

514

THE SYSTEM OF THE WORLD.

But if we now imagine bodies to be projected in the directions of lines
parallel to the horizon from greater heights, as of 5, 10, 100, 1000, or more
miles, or rather as many semi-diameters of the earth, those bodies, accord¬
ing to their different velocity, and the different force of gravity in different
heights, will describe arcs either concentric with the earth, or variously
eccentric, and go on revolving through the heavens in those trajectories,
just as the planets do in their orbs.

As when a stone is projected obliquely, that is, any way but in the per¬
pendicular direction, the perpetual deflection thereof towards the eartli
from the right line in which it was projected is a proof of its gravitation
to the earth, no less certain than its direct descent when only suffered to
fall freely from rest; so the deviation of bodies moving in free spaces from
rectilinear paths, and perpetual deflection therefrom towards any place, is
a sure indication of the existence of some force which from all quarters
impels those bodies towards that place.

And as, from the supposed existence of gravity, it necessarily follows
that all bodies about the earth must press downwards, and therefore must
either descend directly to the earth, if they are let fall from rest, or at
least perpetually deviate from right lines towards the earth, if they are
projected obliquely; so from the supposed existence of a force directed to
any centre, it will follow, by the like necessity, that all bodies upon which
this force acts mast either descend directly to that centre, or at least devi¬
ate perpetually towards it from right lines, if otherwise they should have
moved obliquely in these right lines.

And how from the motions given we may infer the forces, or from the
forces given we may determine the motions, is shewn in the two first Books
of our Principles of Philosophy.

If the earth is supposed to stand still, and the fixed stars to be revolved
in free spaces in the space of 24 hours, it is certain the forces by which
the fixed stars are retained in their orbs are not directed to the earth, but
to the centres of the several orbs, that is, of the several parallel circles
which the fixed stars, declining to one side and the other from the equator,
describe daily; also that by radii drawn to the centres of those orbs the
fixed stars describe areas exactly proportional to the times of description.
Then, because the periodic times are equal (by Cor. Ill, Prop. IY, Book I),
it follows that the centripetal forces are as the radii of the several orbs,
and that they will perpetually revolve in the same orbs. And the like
consequences may be drawn from the supposed diurnal motion of the
planets.

That forces should be directed to no body on which they physically de¬
pend, but to innumerable imaginary points in the axis of the earth, is an
hypothesis too incongruous. It is more incongruous still that those forces
should increase exactly in proportion of the distances from this axis; for

THE SYSTEM OF THE WORLD.

515

this is an indi ation of an increase to immensity, or rather to infinity ;
whereas the forces of natural things commonly decrease in receding from
the fountain from which they flow. But, what is yet more absurd, neither
are the areas described by the same star proportional to the times, nor are
its revolutions performed in the same orb ; for as the star recedes from the
neighbouring pole, both areas and orb increase; and from the increase of
the area it is demonstrated that the forces are not directed to the axis of
the earth. And this difficulty (Cor. 1, Prop. II) arises from the twofold
motion that is observed in the fixed stars, one diurnal round the axis of
the earth, the other exceedingly slow round the axis of the ecliptic. And
the explication thereof requires a composition of forces so perplexed and
so variable, that it is hardly to be reconciled with any physical theory.

That there are centripetal forces actually directed to the bodies of the
sun, of the earth, and other planets, I thus infer.

The moon revolves about our earth, and by radii drawn to its centre
(p. 390) describes areas nearly proportional to the times in which they are
described, as is evident from its velocity compared with its apparent diame¬
ter ; for its motion is slower when its diameter is less (and therefore its
distance greater), and its motion is swifter when its diameter is greater.

The revolutions of the satellites of Jupiter about that planet are more
regular (p. 386); for they describe circles concentric with Jupiter by equa¬
ble motions, as exactly as our senses can distinguish.

And so the satellites of Saturn are revolved about this planet with mo¬
tions nearly (p. 387) circular and equable, scarcely disturbed by any eccen¬
tricity hitherto observed.

That Venus and Mercury are revolved about the sun, is demonstrable
from their moon-like appearances (p. 388); when they shine with a full
face, they are in those parts of their orbs which in respect of the earth lie
beyond the sun ; when they appear half full, they are in those parts whicli
lie over against the sun ; when horned, in those parts which lie between
the earth and the sun ; and sometimes they pass over the sun’s disk, when
directly interposed between the e^rth and the sun.

And Venus, with a motion almost uniform, describes an orb nearly cir¬
cular and concentric with the sun.

But Mercury, with a more eccentric motion, makes remarkable ap¬
proaches to the sun, and goes off again by turns; but it is always swifter
as it is near to the sun, and therefore by a radius drawn to the sun still
describes areas proportional to the times.

Lastly, that the earth describes about the sun, or the sun about the
earth, by a radius from the one to the other, areas exactly proportional to
the times, is demonstrable from the apparent diameter of the sun com¬
pared with its apparent motion.

These are astronomical experiments ; from which it follows, by Prop. I,

516

THE SYSTEM OF THE WORLD.

11, HI, in the first Book of our Principles , and their Corollaries (p. 212,
213, 214). that there are centripetal forces actually directed (either accu¬
rately or without considerable error) to the centres of the earth, of Jupi¬
ter, of Saturn, and of the sun. In Mercury, Venus, Mars, and the lesser
planets, wheie experiments are wanting, the arguments from analogy must
be allowed in their place.

That those forces (p. 212, 213, 214) decrease in the duplicate propor¬
tion of the distances from the centre of every planet, appears by Cor. VI,
Prop. IV, Book 1; for the periodic times of the satellites of Jupiter are
one to another (p. 386, 3S?) in the sesquiplicate proportion of their dis¬
tances from the centre of this planet.

This proportion has been long ago observed in those satellites ; and Mr.
Flcimsted, who had often measured their distances from Jupiter by the
micrometer, and by the eclipses of the satellites, wrote to me, that it holds
to all the accuracy that possibly can be discerned by our senses. And he
sent me the dimensions of their orbits taken by the micrometer, a*nd re¬
duced to the mean distance of Jupiter from the earth, or from the sun,
together with the times of their revolutions, as follows:—

The greatestelon-
gatiouofthesate-
lites from the cen¬
tre of Jupiter as
seen from the sun.
i w w

1st 1 48 or 108
2d 3 01 or 181
3d 4 46 or 286
4th 8 13 h or 493£

The periodic
times of their
revolutions.

4

h

i

n

1

18

28

36

3

13

17

54

7

03

59

36

16

18

5

13

Wherce the sesquiplicate proportion may be easily seen. For example;
the 16^ lS h . 05' 13" is to the time l d . lS h . 28' 36" as 493i" x y 493f ;
to 10S' X \f 10S", neglecting those small fractions which, in observing,
cannot je certainly determined.

Beffl' e the invention of the micrometer, the same distances were deter¬
mined ? \ semi-diameters of Jupiter thus :—

Distance of the

1st

2d

3d

4th

By Galileo, . . .

6

10

16

28

“ Simon Marius

6

10

16

26

u Cassini . . .

5

8

13

23

“ Borelli, more ex¬

actly . . .

5%

8&

14

24%

After the invention of the micrometer :—

By Townley . . .

5,51

8,78

13,47

24,72

11 Flamsted . . .

5,31

8,85

13.98

24,23

More accurately by

the eclipses . .

5,578

8,876

14.159

24,903

THE SYSTEM OF THE WORLD.

617

And the periodic times of those satcllities, by the observations of Mr.
FI a ms ted, are l d . 18 h . 28' 36" | 3 d . 13*. 17' 54" | 7 d . 3 h . 59' 36" | 16 d .
IS h . 5' 13". as above.

And the distances thence computed are 5,578 | S,87S | 14,168 | 24,968,
accurately agreeing with the distances by observation.

Cassini assures us (p. 3SS, 389) that the same proportion is observed
in the circum-saturnal planets. But a longer course of observations is
required before we can have a certain and accurate theory of those planets.

In the circum-solar planets, Mercury and Venus, the same proportion
holds with great accuracy, according to the dimensions of their orbs, as
determined by the observations of the best astronomers.

That Mars is revolved about the sun is demonstrated from the phases
which it shews, and the proportion of its apparent diameters (p. 388, 389,
and 390); for from its appearing full near conjunction with the sun, and
gibbous in its quadratures, it is certain that it surrounds the sun.

And since its diameter appears about five times greater when in opposi¬
tion to the sun than when in conjunction therewith, and its distance from
the earth is reciprocally as its apparent diameter, that distance will be
about five times less when in opposition to than when in conjunction with
the sun; but in both cases its distance from the sun will be nearly about
the same with the distance which is inferred from its gibbous appearance
in the quadratures. And as it encompasses the sun at almost equal dist n-
ces, but in respect of the earth is very unequally distant, so by radii drawn
to the sun it describes areas nearly uniform ; but by radii drawn to the
earth, it is sometimes swift, sometimes stationary, and sometimes retrograde.

That Jupiter, in a higher orb than Mars, is likewise revolved about the
sun, with a motion nearly equable, as well in distance as in the areas des¬
cribed, I infer thus.

Mr. Flamsted assured me, by letters, that all the eclipses of the inner¬
most satellite which hitherto have been well observed do agree with his
theory so nearly, as never to differ therefrom by two minutes of time;
that in the outmost the error is little greater ; in the outmost but one,
scarcely three times greater; that in the innermost but one the difference
is indeed much greater, yet so as to agree as nearly with his computations
as the moon does with the common tables ; and that he computes those
eclipses only from the mean motions corrected by the equation of light dis¬
covered and introduced by Mr. Romer. Supposing, then, that the theory
differs by a less error than that of 2' from the motion of the outmost sat¬
ellite as hitherto described, and taking as the periodic time 16 d . lS h . 5' 13"
to 2■ in time, so is the whole circle or 360° to the arc 1' 48", the error ot
Mr. Flamsted’s computation, reduced to the satellite’s orbit, will be less
than 1' 4S" ; that is, the longitude of the satellite, as seen from fhe centre
of Jupiter, will be determined with a less error than 1' 48". But when

51S

THE SYSTEM OF THE WORLD.

the satellite is in the middle of the shadow, that longitude is the same with
the heliocentric longitude of Jupiter ; and, therefore, the hypothesis which
Mr. Flamsted follows, viz., the Coperniccm , as improved by Kepler , and
('as to the motion of Jupiter) lately corrected by himself, rightly represents
that longitude within a less error than 1' 4S"; but by this longitude, to¬
gether with the geocentric longitude, which is always easily found, the dis¬
tance of Jupiter from the sun is determined ; which must, therefore, be the
very same with that which the hypothesis exhibits. For that greatest error
of 1' 48" that can happen in the heliocentric longitude is almost insensi¬
ble, and quite to be neglected, and perhaps may arise from some yet undis¬
covered eccentricity of the satellite: but since both longitude and distance
are rightly determined, it follows of necessity that Jupiter, by radii drawn
to the sun, describes areas so conditioned as the hypothesis requires, that is.
proportional to the times.

And the same thing may be concluded of Saturn from his satellite, by
the observations of Mr. Huygens and Dr. Halley ; though a longer series
of observations is yet wanting to confirm the thing, and to bring it under
a sufficiently exact computation.

For if Jupiter was viewed from the sun, it would never appear retro¬
grade nor stationary, as it is seen sometimes from the earth, but always to
go forward with a motion nearly uniform (p. 389). And from the very
great inequality of its apparent geocentric motion, we infer (by Prop. Ill
Cor. IV) that the force by which Jupiter is turned out of a rectilinear course,
and made to revolve in an orb, is not directed to the centre of the earth.
And the same argument holds good in Mars and in Saturn. Another centre
of these forces is therefore to be looked for (by Prop. Ii and III, and the
Corollaries of the latter), about which the areas described by radii inter¬
vening may be equable; and that this is the sun, we have proved already
in Mars and Saturn nearly, but accurately enough in Jupiter. It may be
alledged that the sun and planets are impelled by some other force equally
and in the direction of parallel lines ; but by such a force (by Cor. VI of
the Laws of Motion) no change would happen in the situation of the
planets one to another, nor any sensible effect follow: but our business is
with the causes of sensible effects. Let us, therefore, neglect every such
force as imaginary and precarious, and of no use in the phenomena of the
heavens; and the whole remaining force by which Jupiter is impelled will
be directed (by Prop. Ill, Cor. I) to the centre of the sun.

The distances of the planets from the sun come out the same, whether,
with Tycho, we place the earth in the centre of the system, or the sun with
Copernicus : and we have already proved that these distances are true in
Jupiter.

Kepler and Bullialdus have, with great care (p. 3SS), determined the
iistances of the planet 3 from the sun; and hence it is that their tables

THE SYSTEM OF THE WORLD.

519

agree best with the heavens. And in all the planets, in Jupiter and Mar$,
in Saturn and the earth, as well as in Venus and Mercury, the cubes of their
distances are as the squares of their periodic times; and therefore (by Cor.
VI, Prop. IV) the centripetal circum-solar force throughout all the plane¬
tary regions decreases in the duplicate proportion of the distances from the
sun. In examining this proportion, we are to use the mean distances, or
the transverse semi-axes of the orbits (by Prop. XV). and to neglect those
little fractions, which, in defining the orbits, may have arisen from the in¬
sensible errors of observation, or may be ascribed to other causes which we
shall afterwards explain. And thus we shall always find the said propor¬
tion to hold exactly ; for the distances of Saturn, Jupiter, Mars, the Earth,
Venus, and Mercury, from the sun, drawn from the observations of as¬
tronomers, are, according to the computation of Kepler , as the numbers
95 LOGO, 519650, 152350, 100000, 72400, 3SS06; by the computation of
Pullialdus , as the numbers 95419S, 522520, 152350, 100000, 72398,
3S585; and from the periodic times they come out 953S06,520116, 152399,
100000, 72333, 3S710. Their distances, according to Kepler and
Ptdlialdus , scarcely differ by any sensible quantity, and where they
differ most the distances drawn from the periodic times, fall in between them.

That the circum-terrestrial force likewise decreases in the duplicate pro¬
portion of the distances, I infer thus.

The mean distance of the moon from the centre of the earth, is, in semi-
diaincters of the earth, according to Ptolemy , Kepler in his Ephemerides ,
Eidlialdus , Hevelius , and Ricciolus, 59; according to Flamsted , 59^ ;
according to Tycho , 56^• to Vendelin, 60 ; to Copernicus , 60i; to Kir-
cher, 62i ( p . 391, 392, 393).

But Tycho , and all that follow his tables of refraction, making the
refractions of the sun and moon (altogether against the nature of light)
to exceed those of the fixed stars, and that by about four or five minutes
in the horizon, did thereby augment the horizontal parallax of the moon
by about the like number of minutes ; that is, by about the 12th or 15th
part of the whole parallax. Correct this error, and the distance will be¬
come 60 or 61 semi-diameters of the earth, nearly agreeing with what
others have determined.

Let us, then, assume the mean distance of the moon 60 semi-diameters
of the earth, and its periodic time in respect of the fixed stars 27 d . 7 h . 43',
as astronomers have determined it. And (by Cor. VI, Prop. IV) a body
revolved in our air, near the surface of the earth supposed at rest, by
means of a centripetal force which should be to the same force at the dis¬
tance of the moon in the reciprocal duplicate proportion of the distances
from the centre of the earth, that is, as 3600 to 1, would (secluding the
resistance of the air) complete a revolution in l h . 24' 27".

Suppose the circumference of the earth to be 123249600 Paris feet, as

52C

THE SYSTEM OF THE WORLD.

has been determined by the late mensuration of the French (vide p. 406);
then the same body, deprived of its circular motion, and falling by the
impulse of the same centripetal force as before, would, in one second of
time, describe 15^ Paris feet.

This we infer by a calculus formed upon Prop. XXXYI, and it agrees
with what we observe in all bodies about the earth. For by the experi¬
ments of pendulums, and a computation raised thereon, Mr. Huygens has
demonstrated that bodies falling by all that centripetal force with which
(of whatever nature it is) they are impelled near the surface of the earth,
do, in one second of time, describe 15^ Paris feet.

But if the earth is supposed to move, the earth and moon together (by
Cor. IY of the Laws of Motion, and Prop. LYIL will be revolved about
their common centre of gravity. Ana the moon (by Prop. LX) will in
the same periodic time, 27' 1 . 7 h . 43', with the same circum terrestrial force
diminished in the duplicate proportion of the distance, describe an orbit
whose semi-diameter is to the semi-diameter of the former orbit, that is, to
60 semi-diameters of the earth, as the sum of both the bodies of the earth
and moon tc the first of two mean proportionals between this sum and the
body of the earth ; that is, if we suppose the moon (on account of its
mean apparent diameter 31V) t° be about y 1 ^ of the earth, as 43 to

3 _

y/ 42 -f 43| 2 , or as about 12S to 127. And therefore the semi-diameter
of the orbit, that is, the distance between the centres of the moon and
earth, will in this case be 60^ semi-diameters of the earth, almost the same
with that assigned by Copernicus , which the Tychonic observations by no
means disprove ; and, therefore, the duplicate proportion of the decrement
of the force holds good in this distance. I have neglected the increment
of the orbit which arises from the action of the sun as inconsiderable;
but if that is subducted, the true distance will remain about 60|- semi¬
diameters of the earth.

But farther (p. 390); this proportion of the decrement of the forces is
confirmed from the eccentricity of the planets, and the very slow motion
of their apses; for (by the Corollaries of Prop. XLY) in no other pro¬
portion could the circum-solar planets once in every revolution descend to
their least and once ascend to their greatest distance from the sun, and the
places of those distances remain immoveable. A small error from the du¬
plicate proportion would produce a motion of the apses considerable in
every revolution, but in many enormous.

But now, after innumerable revolutions, hardly any such motion has
been perceived in the orbs of the circum-solar planets. Some astronomers
affirm that there is no such motion; others reckon it no greater than what
may easily arise from the causes hereafter to be assigned, and is of no mo¬
ment in the present question.

THE SYSTEM OP THE WORLD.

521

We may even neglect the motion of the moon’s apsis (p. 390, 391), which
is far greater than in the circum-solar planets, amounting in every revolu¬
tion to three degrees; and from this motion it is demonstrable that the
circum-terrestrial force decreases in no less than the duplicate, but far less
than the triplicate proportion of the distance; for if the duplicate propor¬
tion was gradually changed into the triplicate, the motion of the apsis
would thereby increase to infinity; and, therefore, by a very small muta¬
tion, would exceed the motion of the moon’s apsis. This slow motion arises
from the action of the circum-solar force, as we shall afterwards explain.
But, secluding this cause, the apsis or apogeon of the moon will be fixed,
and the duplicate proportion of the decrease of the circum-terrestrial force
in different distances from the earth will accurately take place.

Now that this proportion has been established, we may compare the
forces of the several planets among themselves (p. 391).

In the mean distance of Jupiter from the earth, the greatest elongation
of the outmost satellite from Jupiter's centre (by the observations of Mr.
Flarnsted) is 8' 13"; and therefore the distance of the satellite from the
centre of Jupiter is to the mean distance of Jupiter from tne centre of the
sun as 124 to 52012, but to the mean distance of Venus from the centre
of the sun as L24 to 7234; and their periodic times are 16f d . and 224| d ;
and from hence (according to Cor. II, Prop. IV), dividing the distances by
the squares of the times, we infer that the force by which the satellite is
impelled towards Jupiter is to the force by which Venus is impelled to
wards the sun as 442 to 143; and if we diminish the force by which the
satellite is impelled in the duplicate proportion of the distance 124 to
7231, we shall have the circum-jovial force in the distance of Venus from
the sun to the circum-solar force by which Venus is impelled as T W to
143, or as 1 to 1100; wherefore at equal distances the circum-solar force
is 1100 times greater than the circum-jovial.

And, by the like computation, from the periodic time of the satellite ot
Saturn 15 d . 22 h . and its greatest elongation from Saturn, while that planet
is in its mean distance from us, 3' 20", it follows that the distance of this
satellite from Saturn’s centre is to the distance of Venus from the sun as
92f to 7234; and from thence that the absolute circum-solar force is 2360
times greater than the absolute circum-saturnal.

From the regularity of the heliocentric and irregularity of the geocen¬
tric motions of Venus, of Jupiter, and the other planets, it is evident (by
Cor. IV, Prop. Ill) that the circum-terrestrial force, compared with the cir¬
cum-solar, is very small.

Ricciolus and Vendelin have severally tried to determine the sun’s par¬
allax from the moon’s dichotomies observed by the telescope, and they agree
that it does not exceed half a minute.

Kepler , from Tycho’s observations and his own, found the parallax of

o22

THE SYSTEM OF THE WORLD.

Mars insensible, even in opposition to the sun, when that parallax is some¬
thing greater than the sun’s.

Flamsted attempted the same parallax with the micrometer in the peri-
geon position of Mars, but never found it above 25"; and thence conclud¬
ed the sun’s parallax at most 10".

Whence it follows that the distance of the moon from the earth bears
no greater proportion to the distance of the earth from the sun than 29 to
10000 : nor to the distance of Venus from the sun than 29 to 7233.

From which distances, together with the periodic times, by the method
above explained, it is easy to infer that the absolute circum-soiar force is
greater than the absolute circum-terrestrial force at least 229400 times.

And though we were only certain, from the observations of Ricciolus
and Veudelin , that the sun’s parallax was less than half a minute, yet from
this it will follow that the absolute circum-soiar force exceeds the absolute
circum-terrestrial force S500 times.

By the like computations I happened to discover an analogy, that is ob¬
served between the forces and the bodies of the planets; but, before I ex¬
plain this analogy, the apparent diameters of the planets in their mean
distances from the earth must be first determined.

Mr. Flamsted (p. 387), by the micrometer, measured the diameter of
Jupiter 40" or 41"; the diameter of Saturn’s ring 50" ; and the diameter
of the sun about 32' 13" (p. 387).

But the diameter of Saturn is to the diameter of the ring, according to
Mr. Huygens and Dr. Halley , as 4 to 9; according to Gullet ins , as 4 to
10; and according to Hooke (by a telescope of 60 feet), as 5 to 12. And
from the mean proportion, 5 to 12, the diameter of Saturn’s body is in¬
ferred about 21".

Such as we have said are the apparent magnitudes; but, because of the
unequal refrangibility of light, all lucid points are dilated bv the tele¬
scope, and in the focus of the object-glass possess a circular space whose
breadth is about the 50th part of the aperture of the glass.

It is true, that towards the circumference the light is Su rare as hardly
to move the sense ; but towards the middle, where it is of greater density,
and is sensible enough, it makes a small lucid circle, whose breadth varies
according to the splendor of the lucid point, but is generally about the 3d,
or 4th, or 5th part of the breadth of the whole.

Let ABD represent the circle of the whole light; PQ the small circle
of the denser and clearer light; C the centre of both; CA, CB, semi-di¬
ameters of the greater circle containing a right angle at C; ACBE the
square comprehended under these semi-diameters; AB the diagonal of that
square; EGH an hyperbola with the centre C and asymptotes CA, CB
PG a perpendicular erected from any point P of the line BC, and meeting
the hyperbola in G, and the right lines AB, AE, in K and F : and the

THE SYSTEM OF THE WORLD.

523

density of the light in any place P, will, by my computation, be as the
line FG, and therefore at the centre infinite, but near the circumference
very small. And the whole light within the small circle PQ is to the
without as the area of the quadrilateral figure CAKP to the trian¬

gle PKB. And we are to understand the small circle PQ, to he there
terminated, where FG, the density of the light, begins to be less than what
is required to move the sense.

Hence it was, that, at the distance of 1913S2 feet, a fire of 3 feet in di¬
ameter, through a telescope of 3 feet, appeared to Mr. Picart of S" in
breadth, when it should have appeared only of 3 " 14"'; and hence it is
that the brighter fixed stars appear through the telescope as of 5" or 6" in
diameter, and that with a good full light; but with a fainter light they
appear to run out to a greater breadth. Hence, likewise, it was that He-
velius , by diminishing the aperture of the telescope, did cut off a great part
of the light towards the circumference, and brought the disk of the star to
be more distinctly defined, which, though hereby diminished, did yet ap¬
pear as of 5" or 6" in diameter. But Mr. Huygens , only by clouding the
eye-glass with a little smoke, did so effectually extinguish this scattered
light, that the fixed stars appeared as mere points, void of all sensible
breadth. Hence also it was that Mr. Huygens , from the breadth of bodies
interposed to intercept the whole light of the planets, reckoned their diam¬
eters greater than others have measured them by the micrometer: for the

524

THE SYSTEM OF THE WORLD.

scattered light, which could not be seen before for the stronger light of the
planet, when the planet is hid, appears every way farther spread. "Lastly,
from hence it is that the planets appear so small in the disk of the sun
being lessened by the dilated light. For to Hevelius , Galletius, and Dr.
Halley , Mercury did not seem to exceed 12" or 15"; and Venus appeared
to Mr. Crabtrie only T 3"; to Horrox but 1' 12"; though by the men¬
surations of Hevelius and Hugeuius without the sun’s disk, it ought to
have been seen at least 1' 24". Thus the apparent diameter of the moon,
which in 16S4, a few days both before and after the sun’s eclipse, was
measured at the observatory of Paris 31' 30", in the eclipse itself did not
seem to exceed 30' or 30' 05"; and therefore the diameters of the planets
are to be diminished when without the sun, and to be augmented when
within it, by some seconds. But the errors seem to be less than usual in
the mensurations that are made by the micrometer. So from the diameter
of the shadow, determined by the eclipses of the satellites, Mr. Flamsted
found that the semi-diameter of Jupiter was to the greatest elongation of
the outmost satellite as 1 to 24,903. Wherefore since that elongation is
S' 13", the diameter of Jupiter will be 39^"; and, rejecting the scattered
light, the diameter found by the micrometer 40" or 41" will be reduced to
39^"; and the diameter of Saturn 21" is to be diminished by the like cor¬
rection, and to be reckoned 20", or something less. But (if I am not mis¬
taken) the diameter of the sun, because of its stronger light, is to be dimin¬
ished something more, and to be reckoned about 32', or 32' 6".

That bodies so different in magnitude should come so near to an analogy
with their forces, is not without some mystery (p. 400).

It may be that the remoter planets, for want of heat, have not those me¬
tallic substances and ponderous minerals with which our earth abounds ;
and that the bodies of Venus and Mercury, as they are more exposed to the
sun’s heat, are also harder baked, and more compact.

For, from the experiment of the burning-glass, we see that the heat in¬
creases with the density of light; and this density increases in the recipro¬
cal duplicate proportion of the distance from the sun; from whence the
sun’s heat in Mercury is proved to be sevenfold its heat in our summer
seasons. But with this heat our water boils; and those heavy fluids, quick¬
silver and the spirit of vitriol, gently evaporate, as I have tried by the
thermometer; and therefore there can be no fluids in Mercury but what
are heavy, and able to bear a great heat, and from which substances of great
density may be nourished.

And why not, if God has placed different bodies at different distances
from the sun, so as the denser bodies always possess the nearer places, and
each body enjoys a degree of heat suitable to its condition, and proper for
its nourishment ? From this consideration it will best appear that the
weights of all the planets are one to another as their forces.

THE SYSTEM OF THE WORLD.

525

But I should be glad the diameters of the planets were more accurately
measured ; and that may be done, if a lamp, set at a great distance, is made
to shine through a circular hole, and both the hole and the light of the
lamp are so diminished that the spectrum may appear through the telescope
just like the planet, and may be defined by the same measure: then the
diameter of the hole will be to its distance from the objective glass as the
true diameter of the planet to its distance from us. The light of the lamp
may be diminished by the interposition either of pieces of cloth, or of
smoked glass.

Of kin to the analogy we have been describing, there is another observed
between the forces and the bodies attracted (p. 395, 396, 397). Since the
action of the centripetal force upon the planets decreases in the duplicate
proportion of the distance, and the periodic time increases in the sesquipli-
cate thereof, it is evident that the actions of the centripetal force, and
therefore the periodic times, would be equal in equal planets at equal dis¬
tances from the sun; and in equal distances of unequal planets the total
actions of the centripetal force would be as the bodies of the planets; for
if the actions were not proportional to the bodies to be moved, they could
not equally retract these bodies from the tangents of their orbs in equal
times: nor could the motions of the satellites of Jupiter be so regular, if it
was not that the circum-solar force was equally exerted upon Jupiter and
all its satellites in proportion of their several weights. And the same thing
is to be said of Saturn in respect of its satellites, and of our earth in re¬
spect of the moon, as appears from Cor. II and III, Prop. LXY. And,
therefore, at equal distances, the actions of the centripetal force are equal
upon all the planets in proportion of their bodies, or of the quantities of
matter in their several bodies; and for the same reason must be the same
upon all the particles of the same size of which the planet is composed; for
if the action was greater upon some sort of particles than upon others than
in proportion to their quantity of matter, it would be also greater or less
upon the whole planets not in proportion to the quantity only, but like¬
wise of the sort of the matter more copiously found in one and more
sparingly in another.

In such bodies as are found on our earth of very different sorts, I exam¬
ined this analogy with great accuracy (p. 343, 344).

If the action of the circum-terrestrial force is proportional to the bodies
to be moved, it will (by the Second Law of Motion) move them with equal
velocity in equal times, and will make all bodies let fall to descend through
equal spaces in equal times, and all bodies hung by equal threads to vibrate
in equal times. If the action of the force was greater, the times would be
less; if that was less, these would be greater.

But it has been long ago observed by others, that (allowance being made
for the small resistance of the air) all bodies descend through equal spaces

526

THE SYSTEM OF THE WORLD.

in equal times; and, by the help of pendulums, that equality of times may
be distinguished to great exactness.

I tried the thing in gold, silver, lead, glass, sand, common salt wood,
water, and wheat. I provided two equal wooden boxes. I filled the one
with wood, and suspended an equal weight of gold (as exactly as I could)
in the centre of oscillation of the other. The boxes, hung by equal threads
of 11 feet, made a couple of pendulums perfectly equal in weight and fig¬
ure, and equally exposed to the resistance of the air: and, placing the one
by the other, I observed them to play together forwards and backwards for
a long while, with equal vibrations. And therefore (by Cor. 1 and VI,
Prop. XXIV, Book II) the quantity of matter in the gold was to the quan¬
tity of matter in the wood as the action of the motive force upon all the
gold to the action of the same upon all the wood; that is, as the weight of
the one to the weight of the other.

And by these experiments, in bodies of the same weight, could have dis¬
covered a difference of matter less than the thousandth part of the whole.

Since the action of the centripetal force upon the bodies attracted is, at
equal distances, proportional to the quantities of matter in those bodies,
reason requires that it should be also proportional to the quantity of mat¬
ter in the body attracting.

For all action is mutual, and (p. 83, 93, by the Third Law of Motion)
makes the bodies mutually to approach one to the other, and therefore must
be the same in both bodies. It is true that we may consider one body as
attracting, another as attracted; but this distinction is more mathematical
than natural. The attraction is really common of either to other, and
therefore of the same kind in both.

And hence it is that the attractive force is found in both. The sun at¬
tracts Jupiter and the other planets; Jupiter attracts its satellites; and,
for the same reason, the satellites act as well one upon another as upon Ju¬
piter, and all the planets mutually one upon another.

And though the mutual actions of two planets may be distinguished
and considered as two, by which each attracts the other, yet, as those ac¬
tions are intermediate, they do not make two but one operation between
two terms. Two bodies may be mutually attracted each to the other by
the contraction of a cord interposed. There is a double cause of action,
to wit, the disposition of both bodies, as well as a double action in so far
as the action is considered as upon two bodies; but as betwixt two bodies
it is but one single one. It is not one action by which the sun attracts
Jupiter, and another by which Jupiter attracts the sun ; but it is one ac¬
tion by which the sun and Jupiter mutually endeavour to approach each
the other. By the action with which the sun attracts Jupiter, Jupiter and
the sun endeavours to come nearer together (by the Third Law of Mo¬
tion) ; and by the action with which Jupiter attracts the sun, likewise Ju-

THE SYSTEM OF THE WORLD.

527

piter and the sun endeavor to come nearer together. But the sun is not
attracted towards Jupiter by a twofold action, nor Jupiter by a twofold
action towards the sun ; but it is one single intermediate action, by which
both approach nearer together.

Thus iron draws the load-stone (p. 93), as well as the load-stone
draws the iron; for all iron in the neighbourhood of the load-stone draws
other iron. But the action betwixt the load-stone and iron is single, and
is considered as single by the philosophers. The action of iron upon the
load-stone, is, indeed, the action of the load-stone betwixt itself and the
iron, by which both endeavour to come nearer together: and so it mani¬
festly appears; for if you remove the load-stone, the whole force of the
iron almost ceases.

In this sense it is that we are to conceive one single action to be ex¬
erted betwixt two planets, arising from the conspiring natures of both :
and this action standing in the same relation to both, if it is proportional
to the quantity of matter in the one, it will be also proportional to the
quantity of matter in the other.

Perhaps it may be objected, that, according to this philosophy (p. 39S),
all bodies should mutually attract one another, contrary to the evidence
of experiments in terrestrial bodies; but I answer, that the experiments in
terrestrial bodies come to no account; for the attraction of homogeneous
spheres near their surfaces are (by Prop. LXXII) as their diameters.
Whence a sphere of one foot in diameter, and of a like nature to the
earth, would attract a small body placed near its surface with a force
20000000 times less than the earth would do if placed near its surface;
but so small a force could produce no sensible effect. If two such spheres
were distant but by j of an inch, they would not, even in spaces void uf

A

528

THE SYSTEM OF THE WORLD.

resistance, come together by the force of their mutual attraction in less
than a month’s time; and less spheres will come together at a rate yet
slower, viz., in the proportion of their diameters. Nay, whole mountains
will not be sufficient to produce any sensible effect. A mountain of an
hemispherical figure, three miles high, and six broad, will not, by its at¬
traction, draw the pendulum two minutes out of the true perpendicular;
and it is only in the great bodies of the planets that these forces are to be
perceived, unless we may reason about smaller bodies in manner following.

Let ABCD (p. 93) represent the globe of the earth cut by any plane
AC into two parts ACB, and A CD. The part ACB bearing upon the
part ACD presses it with its whole weight; nor can the part ACD sustain
this pressure and continue unmoved, if it is not opposed by an equal con¬
trary pressure. And therefore the parts equally press each other by their
weights, that is, equally attract each other, according to the third Law of
Motion ; and, if separated and let go, would fall towmrds each other with
velocities reciprocally as the bodies. All which w r e may try and see in the
load-stone, whose attracted part does not propel the part attracting, but is
only stopped and sustained thereby.

Suppose now that ACB represents some small body on the earth’s sur¬
face ; then, because the mutual attractions of this particle, and of the re¬
maining part ACD of the earth towards each other, are equal, but the
attraction of the particle towards the earth (or its weight) is as the matter
of the particle (as we have proved by the experiment of the pendulums),
the attraction of the earth towmrds the particle will likewise be as the
matter of the particle; and therefore the attractive forces of all terres¬
trial bodies will be as their several quantities of matter.

The forces (p. 396), which are as the matter in terrestrial bodies of all
forms, and therefore are not mutable with the forms, must be found in all
sorts of bodies whatsoever, celestial as well as terrestrial, and be in all
proportional to their quantities of matter, because among all there is no
difference of substance, but of modes and forms only. But in the celes¬
tial bodies the same thing is likewise proved thus. We have shewn that
the action of the circum-solar force upon all the planets (reduced to equal
distances) is as the matter of the planets ; that the action of the circum-
jovial force upon the satellites of Jupiter observes the same law ; and the
same thing is to be said of the attraction of all the planets towards every
planet: but thence it follows (by Prop. LXIX) that their attractive forces
are as their several quantities of matter.

As the parts of the earth mutually attract one another, so do those of
all the planets. If Jupiter and its satellites were brought together, and
formed into one globe, without doubt they would continue mutually to
attract cne another as before. And, on the other hand, if the body of
Jupiter was broke into more globes, to be sure, these would no less attract

THE SYSTEM OF THE WORLD.

529

one another than they do the satellites now. From these attractions it is
that the bodies of the earth and all the planets effect a spherical figure, and
their parts cohere, and are not dispersed through the mther. But we have
before proved that these forces arise from the universal nature of matter
(p. 39S), and that, therefore, the force of any whole globe is made up of
the several forces of all its parts. And from thence it follows (by Cor.

III, Prop. LXXIV) that the force of every particle decreases in the dupli¬
cate proportion of the distance from that particle ; and (by Prop. LXXIII
and LXXV) that the force of an entire globe, reckoning from the surface
outwards, decreases in the duplicate, but, reckoning inwards, in the sim¬
ple proportion of the distances from the centres, if the matter of the globe
be uniform. And though the matter of the globe, reckoning from the
centre towards the surface, is not uniform (p. 39S, 399), yet the decrease in
the duplicate proportion of the distance outwards would (by Prop. LXXVI)
take place, provided that difformity is similar in places round about at
equal distances from the centre. And two such globes will (by the same
Proposition) attract one the other with a force decreasing in the duplicate
proportion of the distance between, their centres.

Wherefore the absolute force of every globe is as the quantity of matter
which the globe contains; but the motive force by which every globe is
attracted towards another, and which, in terrestrial bodies, we commonly
call their weight, i3 as the content under the quantities of matter in both
globes applied to the square of the distance between their centres (by Cor.

IV, Prop. LXXVI), to which force the quantity of motion, by which each
globe in a given time will be carried towards the other, is proportional.
And the accelerative force, by which every globe according to its quantity
of matter is attracted towards another, is as the quantity of matter in that
other globe applied to the square of the distance between the centres of
the two (by Cor. II, Prop. LXXVI); to which force, the velocity by which
the attracted globe will, in a given time, be carried towards the other is
proportional. And from these principles well understood, it will be now
easy to determine the motions of the celestial bodies among themselves.

From comparing the forces of the planets one with another, we have
above seen that the circum-solar does more than a thousand times exceed
all the rest; but by the action of a force so great it is unavoidable but that
all bodies within, nay, and far beyond, the bounds of the planetary system
must descend directly to the sun, unless by other motions they are impelled
towards other parts: nor is our earth to be excluded from the number of
3 uch bodies: foy certainly the moon is a body of the same nature with the
planets, and subject to the same attractions with the other planets, seeing
it is by the circum-terrestrial force that it is retained in its orbit. But
that the earth and moon are equally attracted towards the sun, we have
above proved; we have likewise before proved that all bodies are subject to

530

THE SYSTEM OF THE WORLD.

the said common laws of attraction. Nay, supposing any of those bodies
to be deprived of its circular motion about the sun, by having its distance
from the sun, we may find (by Prop. XXXVI) in what space of time it
would in its descent arrive at the sun ; to wit, in half that periodic time in
*vhich the body might be revolved at one half of its former distance; or in
a space of time that is to the periodic time of the planet as 1 to 4^2; as
that Venus in its descent would arrive at the sun in the space of 40 days,
Jupiter in the space of two years and one month, and the earth and moon
together in the space of 66 days and 19 hours. But, since no such thing
happens, it must needs be, that those bodies are moved towards other parts
(p. 75), nor is every motion sufficient for this purpose. To hinder such a
descent, a due proportion of velocity is required. And hence depends the
force of the argument drawn from the retardation of the motions of the
planets. Unless the circum-solar force decreased in the duplicate ratio of
their increasing slowness, the excess thereof would force those bodies to de¬
scend to the sun ; for instance, if the motion (cczteris paribus) was retarded
by one half, the planet would be retained in its orb by one fourth of the
former circum-solar force, and by the excess of the other three fourths
would descend to the sun. And therefore the planets (Saturn, Jupiter,
Mars, Venus, and Mercury) are not really retarded in their perigees, nor
become really stationary, or regressive with slow motions. All these are
but apparent, and the absolute motions, by which the planets continue to
revolve in their orbits, are always direct, and nearly equable. But that
such motions are performed about the sun, we have already proved; and
therefore the sun, as the centre of the absolute motions, is quiescent. For
we can by no means allow quiescence to the earth, lest the planets in their
perigees should indeed be truly retarded, and become truly stationary and
regressive, and so for want of motion should descend to the sun. But
farther; since the planets (Venus, Mars, Jupiter, and the rest) by radi:
drawn to the sun describe regular orbits, and areas (as we have shewii)
nearly and to sense proportional to the times, it follows (by Prop. Ill, and
Cor. Ill, Prop. LXV) that the sun is moved with no notable force, unless
perhaps with such as all the planets are equally moved with, according to
their several quantities of matter, in parallel lines, and so the whole sys¬
tem is transferred in right lines. Reject that translation of the whole
system, and the sun will be almost quiescent in the centre thereof. If the
gun was revolved about the earth, and carried the other planets round about
itself, the earth ought to attract the sun with a great force, but the cir-
cum-solar planets with no force producing any sensible effect, which is
contrary to Cor. Ill, Prop. LXV. Add to this, that if hitherto the earth,
because of the gravitation of its parts, has been placed by most authors in
the lowermost region of the universe; now, for better reason, the sun pos¬
sessed of a centripetal force exceeding our terrestrial gravitation a thousand

THE SYSTEM OF THE WOULD.

53L

times and more, ought t,o be depressed into the lowermost place, and to be
held for the centre of the system. And thus the true disposition of the
whole system will be more fully and more exactly understood.

Because the fixed stars are quiescent one in respect of another (p. 401,
4U2), we may consider the sun, earth, and planets, as one system of bodies
carried hither and thither by various motions among themselves; and the
common centre of gravity of all (by Cor. IV of the Laws of Motion) will
either be quiescent, or move uniformly forward in a right line: in which
case the whole system will likewise move uniformly forward in right lines.
But this is an hypothesis hardly to be admitted; and, therefore, setting it
aside, that common centre will be quiescent: and from it the sun is never
far removed. The common centre of gravity of the sun and Jupiter falls
on (he surface of the sun; and though all the planets were placed towards
the same parts from the sun with Jupiter the common centre of the sun
and all of them would scarcely recede twice as far from the sun’s centre;
and, (herefore, though the sun, according to the various situation of the
planets, is variously agitated, and always wandering to and fro with a slow
motion of libration, yet it never recedes one entire diameter of its own body
from the quiescent centre of the whole system. But from the weights of
the sun and planets above determined, and the situation of all among them¬
selves, their common centre of gravity may be found ; and, this being given,
the sun’s place to any supposed time may be obtained.

About the sun thus librated the other planets are revolved in elliptic
orbits (p 403), and, by radii drawn to the sun, describe areas nearly pro¬
portional to the times, as is explained in Prop. LXV. If the sun was qui¬
escent, and the other planets did not act mutually one upon another, their
orbits would be elliptic, and the areas exactly proportional to the times (by
Prop. XI, and Cor. 1, Prop. XIII). But the actions of the planets among
themselves, compared with the actions of the sun on the planets, are of no
moment, and produce no sensible errors. And those errors are less in rev¬
olutions about the sun agitated in the manner but now described than if
those revolutions were made about the sun quiescent (by Prop. LXV1, and
Cor. Prop. LXVIII), especially if the focus of every orbit is placed in the
common centre of gravity of all the lower included planets; viz., the focus
of the orbit of Mercury in the centre of the sun; the focus of the orbit of
Venus in the common centre of gravity of Mercury and the sun ; the focus
of the orbit of thp earth in the common centre of gravity of Venus, Mer¬
cury, and the sun; and so of the rest. And by this means the foci of the
crbits of all the planets, except Saturn, will not be sensibly removed from
the centre of the sun, nor will the focus of the orbit of Saturn recede sensi¬
bly from the common centre of gravity of Jupiter and the sun. And
therefore astronomers are not far from the truth, when they reckon the
sun’s centre the common focus of all the planetary orbits. In Saturn itself

532

THE SYSTEM CF THE WORLD.

the error thence arising does not exceed 1'45''. And if its orbit, by placing
the focus thereof in the common centre of gravity of Jupiter and the sun,
shall happen to agree better with the phenomena, from thence all that we
have said will be farther confirmed.

If the sun was quiescent, and the planets did not act one on another, the
aphelions and nodes of their orbits would likewise (by Prop. 1, XI, and Cor.
Prop. XIII) be quiescent. And the longer axes of their elliptic orbits
would (by Prop. XV) be as the cubic roots of the squares of their periodic
times : and therefore from the given periodic times would be also given.
But those times are to be measured not from the equinoctial points, which
are moveable, but from the first star of Arias. Put the semi-axis of the
earth's orbit 100000, and the semi-axes of the orbits of Saturn, Jupiter,
Mars, Venus, and Mercury, from their periodic times, will come out
953S06, 520116, 152399, 72333, 38710 respectively. But from the sun’s
motion every semi-axis is increased (by Prop. LX) by about one third of
the distance of the sun’s centre from the common centre of gravity of
the sun and planet (p. 405, 406.) And from the actions of the exterior
planets on the interior, the periodic times of the interior are something
protracted, though scarcely by any sensible quantity; and their aphelions
are transferred (by Cor. VI and VII, Prop. LX VI) by very slow motions
in consequential. And on the like account the periodic times of all, espe¬
cially of the exterior planets, will be prolonged by the actions of the
comets, if any such there are, without the orb of Saturn, and the aphe¬
lions of all will be thereby carried forwards in consequentia. But from
the progress of the aphelions the regress of the nodes follows (by Cor.
XI, XIII, Prop. 1 iXVI). And if the plane of the ecliptic is quiescent, the
l-egress of the nodes (by Cor. XVI, Prop. LXVI) will be to the progress of
the aphelion in every orbit as the regress of the nodes of the moon’s orbit
to the progress of its apogeon nearly, that is, as about 10 to 21. But as¬
tronomical observations seem to confirm a very slow progress of the aphe¬
lions, and a regress of the nodes in respect of the fixed stars. And hence
it is probable that there are comets in the regions beyond the planets, which,
revolving in very eccentric orbs, quickly fly through their perihelion parts,
and, by an exceedingly slow motion in their aphelions, spend almost their
whole time in the regions beyond the planets; as we shall afterwards ex¬
plain more at large.

The planets thus revolved about the sun (p. 413, 414, 415) may at the
same time carry others revolving about themselves as satellites or moons,
as appears by Prop. LXVI. But from the action of the sun our moon
must move with greater velocity, and, by a radius drawn to the earth, de¬
scribe an area greater for the time; it must have its orbit less curve, and
therefore approach nearer to the earth in the syzygies than in the quadra¬
tures, except in so far as the motion of eccentricity hinders those effects.

THE SYSTEM OF THE WORLD.

533

Fcr the eccentricity is greatest when the moon’s apogeon is in the syzygies,
and least when the same is in the quadratures; and hence it is that the
perigeon moon is swifter and nearer to us, but the apogeon moon slower and
farther from us, in the syzygies than in the quadratures. But farther; the
apogeon has a progressive and the nodes a regressive motion, both unequa¬
ble. For the apogeon is more swiftly progressive in its syzygies, more
slowly regressive in its quadratures, and by the excess of its progress above
its regress is yearly transferred in consequentia ; but the nodes are quies¬
cent in their syzygies, and most swiftly regressi ve in their quadratures. But
farther, still, the greatest latitude of the moon is greater in its quadra¬
tures than in its syzygies; and the mean motion swifter in the aphelion of
the earth than in its perihelion. More inequalities in the moon’s motion
have not hitherto been taken notice of by astronomers: but all these fol¬
low from our principles in Cor. II, III, IV, V, VI, VII, VIII, IX, X, XI,
XII, XIII, Prop. LXVI, and are known really to exist in the heavens.
And this may seen in that most ingenious, and if I mistake not, of all, the
most acccurate, hypothesis of Mr. Horrox , which Mr. Flamsted has fitted
to the heavens ; but the astronomical hypotheses are to be corrected in the
motion of the nodes ; for the nodes admit the greatest equation or pros-
thaphaeresis in their octants, and this inequality is most conspicuous when
the moon is in the nodes, and therefore also in the octants: and hence it
was that Tycho, and others after him, referred this inequality to the
octants of the moon, and made it menstrual; but the reasons by us addu¬
ced prove that it ought to be referred to the octants of the nodes, and to
be made annual.

Beside those inequalities taken notice of by astronomers (p. 414, 445,
447,) there are yet some others, by which the moon’s motions are so dis¬
turbed, that hitherto by no law could they he reduced to any certain regu¬
lation. For the velocities or horary motions of the apogee and nodes of
the moon, and their equations, as well as the differe ice betwixt the greatest
eccentricity in the syzygies and the least in the < rrdratures, and that ine¬
quality which we call the variation, in the progress of the year are aug¬
mented and diminished (by Cor. XIV, Prop. LXVI) in the triplicate ratio
of the sun’s apparent diameter. Beside that, the variation is mutable
nearly in the duplicate ratio of the time between the quadratures (by Cor.
I and II, Lem. X, and Cor. XVI, Prop. LXVI); and all those inequali¬
ties are something greater in that part of the orbit which respects the sun
than in the opposite part, but by a difference that is scarcely or not at all
perceptible.

By a computation (p. 422), which for brevity’s sake I do not describe, 1
also find that the area which the moon by a radius drawn to the earth
describes in the several equal moments of time is nearly as the sum of the
number 237 T \, and versed sine of the double distance of the moon from

534

THE SYSTEM OF THE WORLD.

the nearest quadrature in a circle whose radius is unity ; and therefore
that the square of the moon’s distance from the earth is as that sum divid¬
ed by the horary motion of the moon. Thus it is when the variation in
the octants is in its mean quantity ; but if the variation is greater or less,
that versed sine must be augmented or diminished in the same ratio. Let
astronomers try how exactly the distances thus found will agree with the
moon’s apparent diameters.

From the motions of our moon we may derive the motions of the moons
or satellites of Jupiter and Saturn (p. 413); for the mean motion of the
nodes of the outmost satellite of Jupiter is to the mean motion of the nodes
of our moon in a proportion compounded of the duplicate proportion of
the periodic time of the earth about the sun to the periodic time of Jupiter
about the sun, and the simple proportion of the periodic time of the sat¬
ellite about Jupiter to the periodic time of our moon about the earth (by
Cor. XVI, Prop. LXVI): and therefore those nodes, in the space of a hun¬
dred years, are carried S° 24' backwards, or in antecedentia. The mean
motions of the nodes of the inner satellites are to the (mean) motion of
(the nodes of) the outmost as their periodic times to the periodic time of
this, by the same corollary, and are thence given. And the motion of the
apsis of every satellite in consequentia is to the motion of its nodes in
antecedentia , as the motion of the apogee of our moon to the motion of Ps
nodes (by the same Corollary), and is thence given. The greatest equa¬
tions of the nodes and line 9 f the apses of each satellite are to the greatest
equations of the nodes and the line of the apses of the moon respectively
as the motion of the nodes and line of the apses of the satellites in the
time of one revolution of the first equations to the motion of the nodes
and apogeon of the moon in the time of one revolution of the last equa¬
tions. The variation of a satellite seen from Jupiter is to the variation
of our moon in the same proportion as the whole motions of their nodes
respectively, during the times in which the satellite and our moon (after
parting from) are revolved (again) to the sun, by the same Corollary ; and
therefore in the outmost satellite the variation does not exceed 5" 12 ",
From the small quantity of those inequalities, and the slowness of the
motions, it happens that the motions of the satellites are found to be so
regular, that the more modern astronomers either deny all motion to the
nodes, or affirm them to be very slowly regressive.

(P. 404). While the planets are thus revolved in orbits about remote
centres, in the mean time they make their several rotations about their
proper axes; the sun in 26 days; Jupiter in 9 h . 56'; Mars in 24 * h .;
Venus in 23 h .; and that in planes not much inclined to the plane of the
ecliptic, and according to the order of the signs, as astronomers determine
from the spots or macula? that by turns present themselves to our sight in
their bodies; and there is a like revolution of our earth performed in 24' 1 .;

THE SYSTEM OF THE WORLU.

535

and those motions are neither accelerated * nor retarded by the actions of
the centripetal forces, as appears by Cor. XXII, Prop. LXVI ; and there¬
fore of all others they are the most equable and most fit for the mensura¬
tion of time; but those revolutions are to be reckoned equable not from
their return to the sun, but to some fixed star: for as the position of the
planets to the sun is unequably varied, the revolutions of those planets
from sun to sun are rendered unequable.

In like manner is the moon revolved about its axis by a motion most
equable in respect of the fixed stars, viz., in 2? d . 7 h . 43', that is, in the
space of a sidereal month; so that this diurnal motion is equal to the
mean motion of the moon in its orbit; upon which account the same face
of the moon always respects the centre about which this mean motion is
performed, that is, the exterior focus of the moon’s orbit nearly ; and hence
arises a deflection of the moon’s face from the earth, sometimes towards
the east, and other times towards the west, according to the position of the
focus which it respects; and this deflection is equal to the equation of the
moon’s orbit, or to the difference betwixt its mean and true motions; and
this is the moon’s libration in longitude: but it is likewise affected with
a libration in latitude arising from the inclination of the moon’s axis to
the plane of the orbit in which the moon is revolved about the earth; for
that axis retains the same position to the fixed stars nearly, and hence the
poles present themselves to our view by turns, as we may understand from
the example of the motion of the earth, whose poles, by reason of the incli¬
nation of its axis to the plane of the ecliptic, are by turns illuminated by
the sun. To determine exactly the position of the moon’s axis to the
fixed stars, and the variation of this position, is a problem worthy of an
astronomer.

By reason of the diurnal revolutions of the planets, the matter which
they contain endeavours to recede from the axis of this motion; and hence
the fluid parts rising higher towards the equator than about the poles
(p. 405), would lay the solid parts about the equator under water, if those
parts did not rise also (p. 405, 409): upon which account the planets are
something thicker about the equator than about the poles; and their equi¬
noctial points (p. 413) thence become regressive; and their axes, by a
motion of nutation, twice in every revolution, librate towards their eclip¬
tics, and twice return again to their former inclination, as is explained in
Cor. XVIII, Prop. LXVI; and hence it is that Jupiter, viewed through
very long telescopes, does not appear altogether round (p. 409), but having
its diameter that lies parallel to the ecliptic something longer than that
which is drawn from north to south.

And from the diurnal motion and the attractions (p. 415, 418) of the
sun and moon our sea ought twice to rise and twice to fall every day, as
well lunar as solar (by Cor. XIX, XX, Prop. LXVI), and the greatest

536

THE SYSTEM OF THE WORLD.

height of the water to happen before the sixth hour of either day and aftei
the twelfth hour preceding. By the slowness of the diurnal motion the
flood is retracted to the twelfth hour ; and by the force of the motion of
reciprocation it is protracted and deferred till a time nearer to the sixth
hour. But till that time is more certainly determined by the pheno¬
mena, choosing the middle between those extremes, why may we not
conjecture the greatest height of the water to happen at the third hour ?
for thus the water will rise all that time in which the force of the lumi¬
naries to raise it is greater, and will fall all that time in which their force
is less; viz., from the ninth to the third hour when that force is greater,
and from the third to the ninth when it is less. The hours I reckon from
the appulse of each luminary to the meridian of the place, as well under
as above the horizon ; and by the hours of the lunar day I understand the
twenty-fourth parts of that time which the moon spends before it comes
about again by its apparent diurnal motion to the meridian of the place
which it left the day before.

But the two motions which the two luminaries raise will not appear distin¬
guished, but will make a certain mixed motion. In the conjunction or op¬
position of the luminaries their forces will be conjoined, and bring on th€
greatest flood and ebb. In the quadratures the sun will raise the waters
which the moon depresseth, and depress the waters which the moon raiseth;
and from the difference of their forces the smallest of all tides will follow.
And because (as experience tells us) the force of the moon is greater than
that of the sun, the greatest height of the water will happen about the
third lunar hour. Out of the syzygies and quadratures the greatest tide
which by the single force of the moon ought to fall out at the third lunar
hour, and by the single force of the sun at the third solar hour, by the
compounded forces of both must fall out in an intermediate time that ap¬
proaches nearer to the third hour of the moon than to that of the sun;
and, therefore, while the moon is passing from the syzygies to the quadra¬
tures, during which time the third hour of the sun precedes the third of
the moon, the greatest tide will precede the third lunar hour, and that by
the greatest interval a little after the octants of the moon; and by like
intervals the greatest tide will follow the third lunar hour, while the moon
is passing from the quadratures to the syzygies.

But the effects of the luminaries depend upon their distances from the
earth; for when they are less distant their effects are greater, and when
more distant their effects are less, and that in the triplicate proportion of
their apparent diameters. Therefore it is that the sun in the winter time,
being then in its perigee, has a greater effect, and makes the tides in the
syzygies something greater, and those in the quadratures something less,
cwteris panbus , than in the summer season ; and every month the moon
vhile in the perigee, raiseth greater tides than at the distance of 15 days

THE SYSTEM OF THE WORLD.

537

K‘fore or after, when it is in its apogee. Whence it comes to pass that two
Highest tides do not follow one the other in two immediately succeeding
syzygies.

The effect of either luminary doth likewise depend upon its declination
or distance from the equator; for if the luminary w r as placed at the pole,
it would constantly attract all the parts of the waters, without any inten¬
sion or remission of its action, and could cause no reciprocation of motion ;
and, therefore, as the luminaries decline from the equator towards either
pole, they will by degrees lose their force, and on this account will excite
lesser tides in the solstitial than in the equinoctial syzygies. But in the
solstitial quadratures they will raise greater tides than in the quadratures
about the equinoxes ; because the effect of the moon, then situated in the
equator, most exceeds the effect of the sun ; therefore the greatest tides
fall out in those syzygies, and the least in those quadratures, which happen
about the time of both equinoxes ; and the greatest tide in the syzygies is
always succeeded by the least tide in the quadratures, as we find by expe¬
rience. But because the sun is less distant from the earth in winter than
in summer, it comes to pass that the greatest and least tides more fre¬
quently appear before than after the vernal equinox, and more frequently
after than before the autumnal.

Moreover, the effects cf che lumina¬
ries depend upon the latitudes of places.

Let AjdEP represent the earth on all
sides covered with deep w aters; C its ^
centre; P, p, its poles; AE the equa¬
tor; F any place without the equator;

F f the parallel of the place; D d the
correspondent parallel on the other side
of the equator; L the rlaee which the moon possessed three hours before
H the place of the earth directly under it; h the opposite place; K, A*,
the places at 90 degrees distance; CH, Ch, the greatest heights of the sea
from the centre of the earth ; and CK, Cfc, the least heights; and if with
the axes H/?, K/r, an ellipsis is described, and by the revolution cf that
ellipsis about its longer axis H h a spheroid HPKAy?A* is formed, this sphe¬
roid will nearly represent the figure of the sea; and CF, CJ\ CD, Cd, will
represent the sea in the places F,/, D, d. But farther; if in the said revo¬
lution of the ellipsis any point N describes the circle NM, cutting the
parallels F/, Dr/, in any places R, T, and the equator AE in S, CN will
represent the height of the sea in all those places R, S, T, situated in this
circle. Wherefore in the diurnal revolution of any place F the greatest
flood will be in F, at the third hour after the appulse of the moon to the
meridian above the horizon; and afterwards the greatest ebb in Q,, at the
third hour after the setting of the moon ; and then the greatest flood in/

538

THE SYSTEM OF THE WORLD.

at the third Lour after the appulse of the moon to the meridian under tht
horizon , and. lastly, the greatest ebb in Q. at the third hour after the
rising of the moon ; and the latter flood in f will be less than the preced¬
ing flood in F For the whole sea is divided into two huge and hemis¬
pherical floods, one in the hemisphere KH/vC on the north side, the other
in the opposite hemisphere KH/cC, which we may therefore call the north¬
ern and the southern floods: these floods being always opposite the one to
the other, come by turns to the meridians of all places after the interval
of twelve lunar hours; and, seeing the northern countries partake more
of the northern flood, and the southern countries more of the southern
flood, thence arise tides alternately greater and less in all places without
the equator in which the luminaries rke and set. But the greater tide
will happen when the moon declines towards the vertex of the place, about
the third hour after the -appulse of the moon to the meridian above the
horizon; and when the moon changes its declination, that which was the
greater tide will be changed into a lesser; and the greatest difference of
the floods will fall out about the times of the solstices, especially if the
ascending node of the moon is about the first of Aries. So the morning
tides in winter exceed those of the evening, and the evening tides exceed
those of the morning in summer; at Plymouth by the height of one foot,
but at Bristol by the height of 15 inches, according to the observations of
Cidepress and >$ 'turrmj.

But the motions which we have been describing suffer some alteration
from that force of reciprocation which the waters [having once received]
retain a little while by their vis iusita ; whence it comes to pass that the
tides may continue for some time, though the actions of the luminaries
should cease. This power of retaining the impressed motion lessens the
difference of the alternate tides, and makes those tides which immediately
succeed after the svzygies greater, and those which follow next after the
quadratures less. And hence it is that the alternate tides at Plymouth
and Bristol do not differ much more one from the other than bv the height
of a foot, or of 15 inches; and that the greatest tides of all at those ports
are not the first but the third after the svzygies.

And, besides, all the motions are retarded in their passage through shal¬
low channels, so that the greatest tides of all, in some strai s and mouths
of rivers, are the fourth, or even the fifth, after the syzygies.

It may also happen that the greatest tide may be the fourth or fifth
after the syzygies, or fall out yet later, because the motions of the sea are
retarded in passing through shallow places towards the shores; for so the
tide arrives at the western coast of Ireland at the third lunar hour, and an
hour or two after at the ports in the southern coast of the same island; as
also at the islands Cassiterides, commonly Sortings ; then successively at
Falmouth. Plymouth , Portland , the isle of Wight , Winchester , Dover,

THE SYSTEM OF THE WORLD.

539

the mouth of the Thames, and London Bridge, spending twelve hours in
this passage. But farther; the propagation of the tides may be obstructed
even by the channels of the ocean itself, when they are not of depth enough,
for the flood happens at the third lunar hour in the Canary islands; and
at all those western coasts'that lie towards the Atlantic ocean, as of Ire¬
land, France, Spain, and all Africa, to the Cape of Good Hope, except
in some shallow places, where it is impeded, and falls out later; and in the
straits of Gibraltar, where, by reason of a motion propagated from the
Mediterranean sea, it flows sooner. But, passing from those coasts over
the breadth of the ocean to the coasts of America, the flood arrives first at
the most eastern shores of Brazil, about the fourth or fifth lunar hour;
then at the mouth of the river of the Amazons at the sixth hour, but at
the neighbouring islands at the fourth hour; afterwards at the islands of
Bermudas at the seventh hour, and at port St. Augustin in Florida at
seven and a half. And therefore the tide is propagated through the ocean
with a slower motion than it should be according to the course of the
moon; and this retardation is very necessary, that the sea at the same time
may fall between Brazil and Nero France, and rise at the Canary islands,
and on the coasts of Europe and Af rica, and vice versa: for the sea can¬
not rise in one place but by falling in another. And it is probable that
the Pacific sea is agitated by the same laws ; for in the coasts of Chili and
Peru the highest flood is said to happen at the third lunar hour. But
with what velocity it is thence propagated to the eastern coasts of
Japan, the Philippine and other islands adjacent to China, I have not
yet learned.

Farther ; it may happen (p. 418) that the tide may be propagated from
the ocean through different channels towards the same port, and may pass
quicker through some channels than through others, in which case the
same tide, divided into two or more succeeding one another, may compound
new motions of different kinds. Let us suppose one tide to be divided into
two equal tides, the former whereof precedes the other by the space of six
hours, and happens at the third or twenty-seventh hour from the appulse
of the moon to the meridian of the port. If the moon at the time of this
appulse to the meridian was in the equator, every six hours alternately
there would arise equal floods, w r hich, meeting with as many equal ebbs,
would so balance one the other, that, for that day, the water would stag¬
nate, and remain quiet. If the moon then declined from the equator, the
tides in the ocean would be alternately greater and less, as was said; and
from hence two greater and two lesser tides w r ould be alternately propa¬
gated towards that port. But the two greater floods would make the
greatest height of the waters to fall out in the middle time betwixt both,
and the greater and lesser floods would make the waters to rise to a mean
height in the middle time between them; and in the middle time between

O 9

540

THE SYSTEM OF THE WORLD.

the two lesser floods the waters would rise to their least height. Thus in
the space of twenty-four hours the waters would come, not twice, but once
only to their greatest, and once only to their least height; and their great¬
est height, if the moon declined towards the elevated pole, would happen
at the sixth or thirtieth hour after the appulse of the moon to the meridian ;
and when the moon changed its declination, this flood would be changed
into an ebb.

Of all which we have an example in the port of Batsham, in the king¬
dom of Tumptin, in the latitude of 20° 50' north. In that port, on the
day which follows after the passage of the moon over the equator, the
waters stagnate; when the moon declines to the north, they begin to flow'
and ebb, not tw r ice, as in other ports, but once only every day; and the
flood happens at the setting, and the greatest ebb at the rising of the moon.
This tide increaseth with the declination of the moon till the seventh or
eighth day; then for the seventh or eighth day following it decreaseth at
the same rate as it had increased before, and ceaseth when the moon
changeth its declination. After which the flood is immediately changed
into an ebb; and thenceforth the ebb happens at the setting and the flood
at the rising of the moon, till the moon again changes its declination.
There are two inlets from the ocean to this port; one more direct and short
between the island Hainan and the coast of Quantumj, a province of
China ; the other round about between the same island and the coast of
Confirm ; and through the shorter passage the tide is sooner propagated to
Batsham.

In the channels of rivers the influx and reflux depends upon the current
of the rivers, w r hich obstructs the ingress of the waters from the sea, and
promotes their egress to the sea, making the ingress later and slower, and
the egress sooner and faster; and hence it is that the reflux is of longer
duration that the influx, especially far up the rivers, where the force of the
sea is less. So Sturm,y tells us, that in the river Avon , three miles below'
Bristol , the w r ater flows only five hours, but ebbs seven; and without doubt
the difference is yet greater above Bristol, as at Caresham or the Bath.
This difference does likewise depend upon the quantity of the flux and re¬
flux ; for the more vehement motion of the sea near the syzygies of the
luminaries more easily overcoming the resistance of the rivers, will make
the ingress of the water to happen sooner and to continue longer, and will
therefore diminish this difference. But while the moon is approaching to
the syzygies, the rivers will be more plentifully filled, their currents being
obstructed by the greatness of the tides, and therefore will something more
retard the reflux of the sea a little after than a little before the syzygies.
Upon which account the slowest tides of all will not happen in the syzy¬
gies, but precede them a little; and I observed above that the tides before
the syzygies were also retarded by the force of the sun; and from both

THE SYSTEM 0E THE WORLD.

541

causes conjoined the retardation of the tides will be both greater and sooner
before the syzygies. All which I find to be so, by the tide-tables which
Flamsted has composed from a great many observations.

By the laws we have been describing, the times of the tides are governed ;
but the greatness of the tides depends upon the greatness of the seas. Let
C represent the centre of the earth, EADB the oval figure of the seas, OA
the longer semi-axis of this oval, CB the shorter insisting at right angles
upon the former, D the middle point between A and B, and EOF or tCf
the angle at the centre of the earth, subtended by the breadth of the sea
that terminates in the shores E,F, or e,f Now, supposing that the point

A is in the middle between the points E, F, and the point D in the middle
between the points e,f, if the difference of the heights CA, CB, represent
the quantity of the tide in a very deep sea surrounding the whole earth,
the excess of the height CA above the height CE or CF will represent the
quantity of the tide in the middle of the sea EF, terminated by the shores
E, F; and the excess of the height Ce above the height C f will nearly
represent the quantity of the tide on the shores/ of the same sea. Whence
it appears that the tides are far less in the middle of the sea than at the
shores; and that the tides at the shores are nearly as EF (p. 451, 452), the
breadth of the sea not exceeding a quadrantal arc. And hence it is that
near the equator, where the sea between Africa and America is narrow,
the tides are far less than towards either side in the temperate zones, where
the seas are extended wider; or on almost all the shores of the Pacific sea,
as well towards America as towards China, , and within as well as without
the tropics; and that in islands in the middle of the sea they scarcely rise
higher than two or three feet, but on the shores of great continents are
three or four times greater, and above, especially if the motions propagated
from the ocean are by degrees contracted into a narrow space, and the water,
to fill and empty the bays alternately, is forced to flow and ebb with great
violence through shallow places; as Plymouth and Chepstow Bridge in
England , at the mount of St. Michael and town of Avranches in Nor -
maitdy, and at Ccimbaia and Pegu in the East Indies. In which places.

642

THE SYSTEM OF THE WORLD.

the sea, hurried in and out with great violence, sometimes lays the shores
under water, sometimes leaves them dry, for many miles. Nor is the force
of the influx and efflux to be broke till it has raised or depressed the water
to forty or fifty feet and more. Thus also-long and shallow straits that
open to the sea with mouths wider and deeper than the rest of their chan¬
nel (such as those about Britain and the Magellanic Straits at the east¬
ern entry) will have a greater flood and ebb, or will more intend and remit
their course, and therefore will rise higher and be depressed lower. On
the coast of South America it is said that the Pacific sea in its reflux
sometimes retreats two miles, and gets out of sight of those that stand on
shore. Whence in these places the floods will be also higher; but in deepei
waters the velocity of influx and efflux is always less, and therefore tht
ascent and descent is so too. Nor in such places is the ocean known to
ascend to more than six, eight, or ten feet. The quantity of the ascent I
compute in the following manner
Let S represent the sun, T the
earth (419, 420), P the moon,

PAGB the moon’s orbit. In SP
take SK equal to ST and SL to ,

SK in the duplicate ratio of SK
to SP. Parallel to PT draw LM;
and, supposing the mean quantity
of the circum-solar force directed towards the earth to be represented l /
the distance ST or SK, SL will represent the quantity thereof directed
towards the moon. But that force is compounded of the parts SM, LM;
of which the force LM and that part of SM which is represented by TM,
do disturb the motion of the moon (as appears from Prop. LXVT, and its
Corollaries) In so far as the earth and moon are revolved about their
common centre of gravity, the earth will be liable to the action of the like
forces. But we may refer the sums as well of the forces as of the motions
to the moon, and represent the sums of the forces by the lines TM and
ML, which are proportional to them. The force LM, in its mean quan¬
tity, is to the force by which the moon may be revolved in an orbit, about
the earth quiescent, at the distance PT in the duplicate ratio of the moon’s
periodic time about the earth to the earth’s periodic time about the sun
(by Cor. XVII, Prop. LXVI); that is, in the duplicate ratio of 27 d . 7 h .
43' to 365 d . 6 h . 9'; or as 1000 to 1 78725, or 1 to 178§£. The force by
which the moon may be revolved in its orb about the earth in rest, at the
distance PT of 60| semi-diameters of the earth, is to the force by which
it may revolve in the same time at the distance of 60 semi-diameters as
60i to 60; and this force is to the force of gravity with us as 1 to 60 X
60 nearly; and therefore the mean force ML is to the force of gravity at
the surface of the earth as 1 X 60| to 60 X 60 X -17Sf|, or 1 to

THE SYSTEM OF TH2 WORLD.

543

63S092,G. Whence the force TM will be also given from the proportion
of the lines TM, ML. And these are the forces of the sun, by which the
moon’s motions are disturbed.

If from the moon's orbit (p. 449) we descend to the earth’s surface, those
forces will be diminished in the ratio of the distances G0| and 1; and
therefore the force LM will then become 3S604600 times less than the
force of gravity. But this force acting equally every where upon the
earth, will scarcely effect any change on the motion of the sea, and there¬
fore may be neglected in the explication of that motion. The other force
TM, in places where the sun is vertical, or in their nadir, is triple the
quantity of the force ML, and therefore but 12S6S200 times less than the
force of gravity.

Suppose now ADBE to represent the spherical surface of the earth,
crD^E the surface of the water overspreading it, 0 the centre of both, A
the place to which the sun is vertical, B the place opposite; D, E, places
at 90 degrees distance from the former ; ACE'/?*//* a right angled cylindric
canal passing through the earth’s centre. The force TM in any place is
as the distance of the place from the plane DE, on which a line fr^m A
to C insists at right angles, and
therefore in the part of the ca¬
nal which is represented by EC
Ini is of no quantity, but in the
other part AC/A; is as the gravity
at the several heights; for in
descending towards the centre of
the earth, gravity is (by Prop*

LXX11I) every where as the
height; and therefore the force
TM drawing the water upwards
will diminish its gravity in the
leg AC/A; of the canal in a given
ratio : upon which account the
water will ascend in this leg, till its defect of gravity is supplied by its
greater height; nor will it rest in an equilibrium till its total gravity
becomes equal to the total gravity in EC/m, the other leg of the canal.
Because the gravity of every particle is as its distance from the earth’s
centre, the weight of the whole water in either leg will increase in the
duplicate ratio of the height; and therefore the height of the water in the
leg AC/A: will be to the height thereof in the leg C/mE in the subdupli-
cate ratio of the number 12S68201 to 12S68200, or in the ratio of the
number 25623053 to the number 25623052, and the height of the water
in the leg EC/m to the difference of the heights, as 25623052 to 1. But
the height in the leg EC/m is of 19615800 Paris feet, as lias been lately

D

544

THE SYSTEM OF THE WORLD.

found by the mensuration of the French; and, therefore,by the preceding
analogy, the difference of the heights comes out 9} inches of the Paris
foot; and the sun’s force will make the height of the sea at A to exceed
the height of the same at E by 9 inches. And though the water of the
canal ACFnilk be supposed to be frozen into a hard and solid consistence,
yet the heights thereof at A and E, and all other intermediate places, would
still remain the same.

Let A a (in the following figure) represent that excess of height of 9
inches at A, and hf the excess of height at any other place h; and upon
DC let fall the perpendicular /G, meeting the globe of the earth in F ;
and because the distance of the sun is so great that all the right lines
drawn thereto may be considered as parallel, the force TM in any place/
will be to the same force in the place A as the sine FG to the radius AC.

And, therefore, since those forces tend to the sun in the direction of par¬

allel lines, they will generate
the parallel heights Ff } Aa,
in the same ratio; and there¬
fore the figure of the water
T)faeb will be a spheroid
made by the revolution of an
ellipsis about its longer axis
ab. And the perpendicular
height fh will be to the ob¬
lique height F f as/G to /C,
or as FG to AC : and there¬
fore the height fh is to the
height A a in the duplicate

ratio of FG to AC, that is, in the ratio of the versed sine of double the
angle DC/ to double the radius, and is thence given. And hence to the
several moments of the apparent revolution of the sun about the earth we
may infer the proportion of the ascent and descent of the waters at any
given place under the equator, as well as of the diminution of that ascent
and descent, whether arising from the latitude of places or from the sun’s
declination ; viz., that on account of the latitude of places, the ascent and
descent of the sea is in all places diminished in the duplicate ratio of the
co-sines of latitude; and on account of the sun’s declination, the ascent
and descent under the equator is diminished in the duplicate ratio of the
v)-sine of declination. And in places without the equator the half sum
of the morning and evening ascents (that is, the mean ascent) is diminished

nearly in the same ratio.

Let S and L respectively represent the forces of the sun and moon
placed in the equator, and at their mean distances from the earth; R the
radius; T and V the versed sines of double the complements of the sun

THE SYSTEM Of THE WORLD.

545

and moon’s declinations to any given time; D and E the mean apparent
diameters of the sun and moon : and, supposing F and G to be their appa¬
rent diameters to that given time, their forces to raise the tides under the

VG 3 TF 3

equator will be, in the syzygiesL + —S; in the quadratures,
VG 3 TF 3

L — 2R]T^ ^ same rat *° * s likewise observed under

the parallels, from observations accurately made in our northern climates
we may determine the proportion of the forces L and S ; and then by
means of this rule predict the quantities of the tides to every syzygy and
quadrature.

At the mouth of the river Avon, three miles below Bristol (p. 450 to
453), in spring and autumn, the whole ascent of the water in the conjunc¬
tion or opposition of the luminaries (by the observation of Sturr.iy) is
about 45 feet, but in the quadratures only 25. Because the apparent di¬
ameters of the luminaries are not here determined, let us assume them in
their mean quantities, as well as the moon’s declination in the equinoctial
quadratures in its mean quantity, that is, 23|°; and the versed sine of
double its complement will be 1082, supposing the radius to be 1000. But
the declinations of the sun in the equinoxes and of the moon in the syzy-
gies are of no quantity, and the versed sines of double the complements
are each. 2000. Whence those forces become L + S in the syzygies, and
1682

L — S in the quadratures; respectively proportional to the heights

of the tides of 45 and 25 feet, or of 0 and 5 paces. And, therefore, mul-

, , , , 15138 ,

tiplying the extremes and the means, we have 5L 4- oS = T^Trrrr L —

1 J n ’ 2000

2S000 0 „

9S, or L = S = 5 t 5 t S.

But farther; I remember to have been told that in summer the ascent of
the sea in the syzygies is to the ascent thereof in the quadratures as about
5 to 4. In the solstices themselves it is probable that the proportion may
be something less, as about 6 to 5; whence it would follow that L is =

1682

5|S [for then the proportion is

L +

1682

2000

S :L

1682

2000

,S : : 6 : 5].

Till we can more certainly determine the proportion from observation, let
us assume L = 5iS; and since the heights of the tides are as the forces
which excite them, and the force of the sun is able to raise the tides to the
height of nine inches, the moon’s force will be sufficient to raise the same
to the height of four feet. And if we allow that this height may be
doubled, or perhaps tripled, by that force of reciprocation which we observe
in the motion of the waters, and by -which their motion once be^un is kept

35

546

THE SYSTEM OF THE WORLD.

up for some time, there will be force enough to generate all that quantity
of tides which we really lind in the ocean.

Thus we have seen that these forces are sufficient to move the sea. But,
so far as I can observe, they will not be able to produce any other effect
sensible on our earth; for since the weight of one grain in 4000 is not
sensible in the nicest balance: and the sun’s force to move the tides is
1286S200 less than the force of gravity ; and the sum of the forces of both
moon and sun, exceeding the sun’s force only in the ratio of 6} to 1, is still
2032890 times less than the force ol gravity ; it is evident that both forces
together are 500 times less than what is required sensibly to increase < r
diminish the weight of any body in a balance. And, therefore, they will
not sensibly move any suspended body ; nor will they produce any sensible
effect on pendulums, barometers, bodies swimming in stagnant water, or in
the like statical experiments. In the atmosphere, indeed, they will excite
such a flux and reflux as they do in the sea, but with so small a motion
that no sensible wind will be thence produced.

If the effects of both moon and sun in raising the tides (p. 45 4), as well
as their apparent diameters, were equal among themselves, their absolute
forces would (by Cor. XIV, Prop. LXV1) be as their magnitudes. But the
effect of the moon is to the effect of the sun as about 5j to 1; and the
moon’s diameter less than the sun's in the ratio of 31^ to 32y, or of 45 to
46. Now the force of the moon is to be increased in the ratio of the effect
directly, and in the triplicate ratio of the diameter inversely. Whence the
force of the moon compared with its magnitude will be to the force of the
sun compared with its magnitude in the ratio compounded of 5£ to 1, and
the triplicate of 45 to 46 inversely, that is, in the ratio of about 5 r \ to 1.
And therefore the moon, in respect of the magnitude of its body, has au
absolute centripetal force greater than the sun in respect of the magnitude
of its body in the ratio to 5 r \ to 1, and is therefore more dense in the
same ratio.

In the time of 2 7 d . 7 h . 43', in which the moon makes its revolution about
the earth, a planet may be revolved about the sun at the distance of 18,95 1
diameters of the sun from the sun’s centre, supposing the mean apparen
diameter of the sun to be 32}'; and in the same time the moon may be re¬
volved about the earth at rest, at the distance of 30 of the earth’s diame¬
ters. If in both cases the number of diameters was the same, the absolute
circum-terrestrial force would (by Cor. II. Prop. LXX11) be to the absolute
circum-solar force as the magnitude of the earth to the magnitude of the
cun. Because the number of the earth’s diameters is greater in the ratio
of 30 to 18,954,"the body of the earth will be less in the triplicate of that
ratio, that is, in the ratio of 3§f to 1. Wherefore the earth’s force, for the
magnitude of its body, is to the sun’s force, for the magnitude of its body,
as 3|f to 1: and consequently the earth’s density to the sun’s will be iL

THE SYSTEM OF THE WORLD

54 7

the same ratio. Since, then, the moon’s density is tu the sun’s density as
5 v 7 ff to 1, the moon’s density will be to the earth’s density as 5 T \ to 3§f,
or as 23 to 16. Win refore since the moon’s magnitude is to the earth’s
magnitude as about l to 4H, the moon’s absolute centripetal force will be
to the earth’s absolute centripetal force as about 1 to 29, and the quantity
of matter in the moon to the quantity of matter in the earth in the same
ratio. And hence the common centre of gravity of the earth and moon is
more exactly determined than hitherto has been done; from the knowledge
of which we may now T infer the moon’s distance from the earth with greater
accuracy. But I would rather wait till the proportion of the bodies of the
moon and earth one to the other is more exactly defined from the phso
nomena of the tides, hoping that in the mean time the circumference of the
earth may be measured from more distant stations than any body has yet
employed for this purpose.

Thus I have given an account of the system of the planets. As to the
fixed stars, the smallness of their annual parallax proves them to be re¬
moved to immense distances from the system of the planets: that this
parallax is less than one minute is most certain ; and from thence it follows
that the distance of the fixed stars is above 360 times greater than the
distance of Saturn from ;he sun. Such as reckon the earth one of the
planets, and the sun one of the fixed stars, may remove the fixed stars to
yet greater distances by the following arguments: from the annual motion
of the earth there would happen an apparent transposition of the fixed
stars, one in respect of another, almost equal to their double parallax: but
the greater and nearer stars, in respect of the more remote, which are only
seen by the telescope, have not hitherto been observed to have the least
motion. If we should suppose that motion to be but less than 20", the
distance of the nearer fixed stars would exceed the mean distance of Saturn
by above 2000 times. Again; the disk of Saturn, which is only 17" or
18" in diameter, receives but about IT o ooVoo o o °f ^ ie sun’s light; for so
much less is that disk than the whole spherical surface of the orb of Saturn.
Now if we suppose Saturn to reflect about ± of this light, the whole light
reflected from its illuminated hemisphere w r ill be about 4 2 ?o oV 0 00 0 °f the
whole light emitted from the sun’s hemisphere; and, therefore, since light
is rarefied in the duplicate ratio of the distance from the luminous body, if
the sun was 10000 v/42 times more distant than Saturn, it would yet ap¬
pear as lucid as Saturn now does without its ring, that is, something more
lucid than a fixed star of the first magnitude. Let us, therefore, suppose
that the distance from wdiich the sun would shine as a fixed star exceeds
that of Saturn by about 100,000 times, and its apparent diameter will be
7 V . 16 vi . and its parallax arising from the annual motion of the earth 13"" :
and so great will be the distance, the apparent diameter, and the parallax
of the fixed stars of the first magnitude, in bulk and light equal to our sun.

THE SYSTEM OF THE WORLD.

48

Some may, perhaps, imagine that a great part of the light of the fixed stars
is intercepted and lost in its passage through so vast spaces, and upon that
account pretend to place the fixed stars at nearer distances; but at this
rate the remoter stars could be scarcely seen. Suppose, for example, that
£ of the light perish in its passage from the nearest fixed stars to us; then
£ will twice perish in its passage through a double space, thrice through a
triple, and so forth. And, therefore, the fixed stars that are at a double
distance will be 16 times more obscure, viz., 4 times more obscure on ac¬
count of the diminished apparent diameter; and, again, 4 times more on
account of the lost light. And, by the same argument, the fixed stars at a
triple distance will be 9 X 4 X 4, or 144 times more obscure; and those
at a quadruple distance will be 16 X 4 X 4 X 4, or 1024 times more ob¬
scure ; but so great a diminution of light is no ways consistent with the
phenomena and with that hypothesis which places the fixed stars at differ¬
ent distances.

Tne fixed stars being, therefore, at such vast distances from one another
(p. 460, 461), can neither attract each other sensibly, nor be attracted by
our sun. But the comets must unavoidably be acted on by the circum¬
solar force; for as the comets were placed by astronomers above the moon,
because they were found to have no diurnal parallax, so their annual
parallax is a convincing proof of their descending into the regions of the
planets. For all the comets which move in a direct course, according to
the order of the signs, about the end of their appearance become more than
ordinarily slow, or retrograde, if the earth is between them and the sun;
and more than ordinarily swift if the earth is approaching to a heliocen¬
tric opposition with them. Whereas, on the other hand, those which move
against the order of the signs, towards the end of their appearance, appear
swifter than they ought to be if the earth is between them and the sun ;
and slower, and perhaps retrograde, if the earth is in the other side of its
erbit. This is occasioned by the motion of the earth in different’situa¬
tions. If the earth go the same way with the comet, with a swifter
motion, the comet becomes retrograde; if with a slower motion, the comet
becomes slower, however ; and if the earth move the contrary way, it be¬
comes swifter; and by collecting the differences between the slower and
swifter motions, and the sums of the more swift and retrograde motions,
and comparing them with the situation and motion of the earth from*
whence they arise, I found, by means of this parallax, that the distances
of the comets at the time they cease to be visible to the naked eye are
always less than the distance of Saturn, and generally even less than the
distance of Jupiter.

The same thing may be collected from the curvature of the way of the
comets (p. 462). These bodies go on nearly in great circles while their
motion continues swift; but about the end of their course, when that part

THE SYSTEM OF THE WORLD.

549

of their apparent motion which arises from the parallax bears a greater
proportion to their whole apparent motion, they commonly deviate from
those circles; and when the earth goes to one side, they deviate to the
other; and this deflection, because of its corresponding with the motion
of the earth, must arise chiefly from the parallax; and the quantity there¬
of is so considerable, as, by my computation, to place the disappearing
comets a good deal lower than Jupiter. Whence it follows, that, when
they approach nearer to its in their perigees and perihelions, they often de¬
scend below the orbits of Mars and the inferior planets.

Moreover, this nearness of the comets is confirmed by the annual paral¬
lax of the orbit, in so far as the same is- pretty nearly collected by the
supposition that the comets move uniformly in right lines. The method
of collecting the distance of a comet according to this hypothesis from
four observations (first attempted by Kepler, and perfected by Dr. Wallis
and Sir Christopher Wren) is well known; and the comets reduced to
this regularity generally pass through the middle of the planetary region.
So the comets of the year 1607 and 1618, as their motions are defined by
Kepler, passed between the sun and the earth ; that of the year 16 *4 be¬
low the orbit of Mars; and that in 16S0 below the orbit of Mercury, as
its motion was defined by Sir Christopher Wren and others. By a like,
rectilinear hypothesis, Hevelins placed all the comets about which we have
any observations below the orbit of Jupiter. It is a false notion, there¬
fore, and contrary to astronomical calculation, which some have enter¬
tained, who, from the regular motion of the comets, either remove them
into the regions of the fixed stars, or deny the motion of the earth ; where¬
as their motions cannot be reduced to perfect regularity, unless we suppose
them to pass through the regions near the eartli in motion ; and these are
the arguments drawn from the parallax, so far as it can be determined
without an exact knowledge of the orbits and motions of the comets.

The near approach of the comets is farther confirmed from the light of
their heads (p. 463, 465); for the light of a celestial body, illuminated by
the sun, and receding to remote parts, is diminished in the quadruplicate
proportion of the distance; to wit, in one duplicate proportion on account
of the increase of the distance from the sun ; and in another duplicate
proportion on account of the decrease of the apparent diameter. Hence it
may be inferred, that Saturn being at a double distance, and having its
apparent diameter nearly half of that of Jupiter, must appear about 16
times more obscure; and that, if its distance were 4 times greater, its
light would be 256 times less; and therefore would be hardly perceivable
to the naked eye. But now the comets often equal Saturn’s light, without
exceeding him in their apparent diameters. So the comet of the year
1668, according to Dr. Hooke's observations, equalled in brightness the
light of a fixed star of the first magnitude; and its head, or the star in

550

THE SYSTEM OF THE WORLD.

the middle of the coma, appeared, through a telescope oi 15 feet, as lucid
as Saturn near the horizon ; but the diameter of the head was only 25"
that is, almost the same with the diameter of a circle equal to Saturn
and his ring. The coma or hair surrounding the head was about ten times
as broad; namely, 4| min. Again ; the least diameter of the hair of the
comet of the year 16S2, observed by Mr. Flamsted with a tube of 16 feet
and measured with the micrometer, wa3 2' 0" ; but the nucleus, or star in
the middle, scarcely possessed the tenth part of this breadth, and was
therefore only 11 " or 12 " broad; but the light and clearness of its head
exceeded that of the year 1680, and was equal to that of the stars of the
first or second magnitude. Moreover, the comet of the year 1665, in April,
as Hevelius informs us, exceeded almost all the fixed stars in splendor, and
even Saturn itself, as being of a much more vivid colour; for this comet
was more lucid than that which appeared at the end of the foregoing year
and was compared to the stars of the first magnitude. The diameter of
the coma was about 6'; but the nucleus, compared with the planets by
means of a telescope, was plainly less than Jupiter, and was sometimes
thought less, sometimes equal to the body of Saturn within the ring. To
this breadth add that of the ring, and the whole face of Saturn will be
twice as great as that of the comet, with a light not at all more intense;
and therefore the comet was nearer to the sun than Saturn. From the
proportion of the nucleus to the whole head found by these observations,
and from its breadth, which seldom exceeds 8 ' or 12 ', it appears that the
stars of the comets are most commonly of the same apparent magnitude
as the planets ; but that their light may be compared oftentimes with that
of Saturn, and sometimes exceeds it. And hence it is certain that in their
perihelia their distances can scarcely be greater than that of Saturn. At
twice that distance, the light would be four times less, which besides by its
dim paleness would be as much inferior to the light of Saturn as the light
of Saturn is to the splendor of Jupiter : but this difference would be easily
observed. At a distance ten times greater, their bodies must be great*, r
than that of the sun; but their light would be 100 times fainter than
that of Saturn. And at distances still greater, their bodies would far
exceed the sun ; but, being in such dark regions, they must be no longer
visible. So impossible is it to place the comets in the middle regions be¬
tween the sun and fixed stars, accounting the sun as one of the fixed stars;
for certainly they would receive no more light there from the sun than we
do from the greatest of the fixed stars.

So far'we have gone without considering that obscuration which comets
suffer from that plenty of thick smoke which encompasseth their heads,
and through which the heads always shew dull as through a cloud ; for
by how much the more a body is obscured by this smoke, by so much tbs
more near it must be allowed to come to the sun, that it may vie with the

THE SYSTEM OF THE WORLD.

551

planets in the quantity of light which it reflects; whence it is probable
that the comets descend far below the orbit of Saturn, as we proved before
frcm their parallax. But, above all, the thing is evinced from their tails,
which must be owing either to the sun’s light reflected from a smoke
arising from them, and dispersing itself through the aether, or to the light
of their own heads.

In the former case we must shorten the distance of the comets, lest we be
obliged to allow that the smoke arising from their heads is propagated
through such a vast extent of space, and with such a velocity of expansion,
as will seem altogether incredible; in the latter case the whole light of
both head and tail must be ascribed to the central nucleus. But, then, if
we suppose all this light to be united and condensed within the disk of the
nucleus, certainly the nucleus will by far exceed Jupiter itself in splendor,
especially when it emits a very large and lucid tail. If, therefore, under a less
apparent diameter, it reflects more light, it must be much more illuminated
by the sun, and therefore much nearer to it. So the comet that appeared
Dec. 12 and 15, O.S. Anno 1679, at the time it emitted a very shining
tail, whose splendor was equal to that of many stars like Jupiter, if their
light were dilated and spread through so great a space, was, as to the mag¬
nitude of its nucleus, less than Jupiter (as Mr. Flaw sled, observed), and
therefore was much nearer to the sun: nay, it was even less than Mercury.
For on the 17th of that month, when it was nearer to the earth, it ap¬
peared to Cassini through a telescope of 35 feet a little less than the globe
of Saturn. On the 8th of this month, in the morning, Dr. Halley saw the
tail, appearing broad and very short, and as if it rose from the body of the
sun itself, at that time very near its rising. Its form was like that of an
extraordinary bright cloud; nor did it disappear till the sun itself began
to be seen above,,the horizon. Its splendor, therefore, exceeded the light of
the clouds till the sun rose, and far surpassed that of all the stars together,
as yielding only to the immediate brightness of the sun itself. Neither
Mercury, nor Venus, nor the moon itself, are seen so near the rising sun.
Imagine all this dilated light collected together, and to be crowded into
the orbit of the comet’s nucleus which was less than Mercury; by its
splendor, thus increased, becoming so much more conspicuous, it will vastly
exceed Mercury, and therefore must be nearer to the sun. On the 12th
and 15th of the same month, this tail, extending itself over a much greater
space, appeared more rare; but its light was still so vigorous as to become
visible when the fixed stars were hardly to be seen, and soon after to appear
like a fiery beam shining in a wonderful manner. From its length, which
was 40 or 50 degrees, and its breadth of 2 degrees, we may compute what
the light of the whole must be

This near approach of the comets to the sun is confirmed from the situ-
tion they are seen in when their tails appear most resplendent; for when

(jo 2

THE SYSTEM OF THE WORLD.

the head passes by the sun, and lies hid under the solar rays, very bright
and shining ta Is, like fiery beams, are said to issue from the horizon; but
afterwards, when the head begins to appear, and is got farther from the
sun, that splendor always decreases, and turns by degrees into a paleness
like to that of the milky way, but much more sensible at first; after that
vanishing gradually. Such was that most resplendent comet described by
Aristotle , Lib. 1, Meteor. 6. “The head thereof could not be seen, because
it set before the sun, or at least was hid under the sun’s rays; but the next
day it was seen as well as might be; for, having left the sun but a very
little way, it set immediately after it; and the scattered light of the head
obscured by the too great splendour (of the tail) did not yet appear. But
afterwards (says Aristotle), when the splendour of the tail was now dimin¬
ished (the head of), the comet recovered its native brightness. And the
splendour of its tail reached now to a third part of the heavens (that is, to
60°). It appeared in the winter season, and, rising to Orion's girdle, there
vanished away.” Two comets of the same kind are described by Justin ,
Lib. 37, which, according to his account, “shined so bright, that the whole
heaven seemed to be on fire; and by their greatness filled up a fourth part
of the heavens, and by their splendour exceeded that of the sun.” By
which last words a near position of these bright comets and the rising or
setting sun is intimated (p. 494, 495). We may add to these the comet of
the year 1101 or 1106, “the star of which was small and obscure (like that
of 16S0); but the splendour arising from it extremely bright, reaching like
a fiery beam to the east and north,” as Hevelius has it from Simeon , the
monk of Durham. It appeared at the beginning of February about the
evening in the south-west. From this and from the situation of the tail
we may infer that the head was near the sun. Matthew Paris says, “it
was about one cubit from the sun; from the third [or rather the sixth] to
the ninth hour sending out a long stream of light.” The comet of 1264,
in July, or about the solstice, preceded the rising sun, sending out its beams
with a great light towards the west as far as the middle of the heavens;
and at the beginning it ascended a little above the horizon : but as the sun
went forwards it retired every day farther from the horizon, till it passed
by the very middle of the heavens. It is said to have been at the beginning
large and bright, having a large coma, which decayed from day to day. It
is described in Append. Matth. Paris, Hist. Ang. after this manner: “A;/.
Christi 1265, there appeared a comet so wonderful, that none then living
had ever seen the like; for, rising from the east with a great brightness, it
extended itself with a great light as far as the middle of the hemisphere
towards the w'est.” The Latin original being somewhat barbarous and ob¬
scure, it is here subjoined. Ah orieute enim cum magno fulgore sur -
i--‘is, usque ad medium hemisphcerii versus occidentem, omnia per lucid*
pertrahebai.

THE SYSTEM OF THE WORLD.

553

il In the year 1401 or 1402, the sun being got below the horizon, there
appeared in the west a bright and shining comet, sending out a tail up¬
wards, in splendor like a flame of fire, and in form like a spear, darting its
rays from west to east. When the sun was sunk below the horizon, by the
lustre of its own rays it enlightened all the borders of the earth, not per¬
mitting the other stars to shew their light, or the shades of night to darken
the air, because its light exceeded that of the others, and extended itself to
the upper part of the heavens, flaming/ 7 &c., Hist. Byzant. Due. Mich.
Nepot. From the situation of the tail of this comet, and the time of its
first appearance, we may infer that the head was then near the sun, and
went farther from him every day; for that comet continued three months.
In the year 1527, Aug. 11, about four in the morning, there was seen al¬
most throughout Europe a terrible comet in Leo , which continued flaming
an hour and a quarter every day. It rose from the east, and ascended to
the south and west to a prodigious length. It was most conspicuous to the
north, and its cloud (that is, its tail) was very terrible; having, according
to the fancies of the vulgar, the form of an arm a little bent holding a
sword of a vast magnitude. In the year 1618, in the end of November ,
there began a rumour, that there appeared about sun-rising a bright beam,
which was the tail of a comet whose head was yet concealed within the
brightness of the solar rays. On Nov. 24, and from that time, the comet
itself appeared with a bright light, its head and tail being extremely re¬
splendent. The length of the tail, which was at first 20 or 30 deg., in¬
creased till December 9, when it arose to 75 deg,, but with a light much
more faint and dilute than at the beginning. In the year 1668, March 5,
N. S., about 7 in the evening, P. Valent. Estancius, being in Brazil , saw
a comet near the horizon in the south-west. Its head was small, and
scarcely discernible, but its tail extremely bright and refulgent, so that the
reflection of it from the sea was easily seen by those who stood upon the
shore. This great splendor lasted but three days, decreasing very remark¬
ably from that time. The tail at the beginning extended itself from west
to south, and in a situation almost parallel to the horizon, appearing like
a shining beam 23 deo\ in length. Afterwards, the light decreasing, its
magnitude increased till the comet ceased to be visible; so that Cassini ,
at Bologna , saw it {Mar. 10, 11, 12) rising from the horizon 32 deg. in
length. In Portugal it is said to have taken up a fourth part of the
heavens (that is, 45 deg.), extending itself from west to east with a notable
brightness; though the whole of it was not seen, because the head in this
part of the world always lay hid below the horizon. From the increase of
the tail it is plain that the head receded from the sun, and was nearest to
it at the beginning, when the tail appeared brightest.

To all these we may add the comet of 1680, whose wonderful splendor
at the conjunction of the head with the sun was above described. But sc

554

THE SYSTEM OF THE WORLD.

great a splendor argues the comets of this kind to have really passed near
the fountain of light, especially since the tails never shine so much in
their opposition to the sun; nor do we read that hery beams have ever ap¬
peared there.

i iastly, the same thing is inferred (p. 466, 467) from the light of the
heads increasing in the recess of the comets from the earth towards the
sun, and decreasing in their return from the sun towards the earth; for so
the last comet of the year 1665 (by the observation of Hevelius ), from the
time that it was first seen, was always losing of its apparent motion, and
therefore had already passed its perigee: yet the splendor of its head was
daily increasing, till, being hid by the sun’s rays, the comet ceased to ap¬
pear. The comet of the year 1683 (by the observation of the same He-
jelius), about the end of July , when it first appeared, moved at a very
slow rate, advancing only about 40 or 45 minutes in its orbit in a day’s
time. But from that time its diurnal motion was continually upon the
increase till September 4, when it arose to about 5 degrees ; and therefore
in all this interval of time the comet was approaching to the earth. Which
is likewise proved from the diameter of its head measured with a microme¬
ter ; for, August the 6th, Hevelius found it only 6' 5", including the
coma; which, September 2, he observed 9' 7". And therefore its head
appeared far less about the beginning than towards the end of its motion,
though about the beginning, because nearer to the sun, it appeared far
more lucid than towards the end, as the same Hevelius declares. Where¬
fore in all this interval of time, on account of its recess from the sun,
it decreased in splendor, notwithstanding its access towards the earth. The
comet of the year 1618, about the middle of December , and that of the
year 1680, about the end of the same month, did both move with their
greatest velocity, and were therefore then in their perigees ; but the greatest
splendor of their heads was seen two weeks before, when they had jdst got
clear of the sun’s rays; and the greatest splendor of their tails a little
more early, when yet nearer to the sun. 'The head of the former comet,
according to the observations of Cysatus, Dec . 1, appeared greater than
the stars of the first magnitude; and, Dec. 16 (being then in its perigee),
of a small magnitude, and the splendor or clearness was much diminished.
Jan. 7, Kepler , being uncertain about the head, left off observing. Dec .
12, the head of the last comet was seen and observed by Fla ms ted at the
distance of 9 degrees from the sun, which a star of the third magnitude
could hardly have been. December 15 and 17, the same appeared like a
star of the third magnitude, its splendor being diminished by the bright
clouds near the setting sun. Dec. 26, when it moved with the greatest
swiftness, and was almost in its perigee, it was inferior to Os Pegasi, a
3tar of the third magnitude. Jan. 3, it appeared like a star of the fourth ;
fan. 9, like a star of the fifth. Jan. 13, it disappeared, by reason of tb*

/

THE SYSTEM OF THE WORLD. 555

brightness of the moon, which was then in its increase. Jan, 25, it was
scarcely equal to the stars of the seventh magnitude. If we take equal
dines on each hand of the perigee, the heads placed at remote distances
would have shined equally before and after, because of their equal distances
from the earth. That in one case they shined very bright, and in the
other vanished, is to be ascribed to the nearness cf the sun in the first case,
and his distance in the other; and from the great difference of the light
in these two cases we infer its great nearness in the first of them ; for
the light of the comets uses to be regular, and to appear greatest when
their heads move the swiftest, and are therefore in their perigees, except¬
ing in so far as it is increased by their nearness to the sun.

From these things I at last discovered why the cornets frequent so much
the region of the sun. If they were to be seen in the regions a great w*ay
beyond Saturn, they must appear oftener in these parts of the heavens
that are opposite to the sun ; for those which are in that situation would
be nearer to the earth, and the interposition of the sun would obscure tire
others: but, looking over the history of comets, I find that four or five
times more have been seen in the hemisphere towards the sun than in the
opposite hemisphere; besides, without doubt, not a few which have been
hid by the light of the sun; fur comets descending into our parts neither
emit tails, nor are so well illuminated by the sun, as to discover them¬
selves to our naked eyes, till they are come nearer to us than Jupiter. But
the far greater part of that spherical space, which is described about the
sun with so small an interval, lies on that side of the earth which regards
the sun, and the comets in that greater part are more strongly illuminated,
as being for the most part nearer to the sun : besides, from the remarka¬
ble eccentricity of their orbits, it comes to pass that their lower apsides
are much nearer to the sun than if their revolutions were performed in
circles concentric to the sun.

Hence also we understand why the tails of the comets, while their heads
are descending towards the sun, always appear short and rare, and are sel¬
dom said to have exceeded 15 or 20 deg. in length; but in the recess of
the heads from the sun often shine like fiery beams, and soon after reach
to 40, 50, 60, 70 deg. in length, or more. This great splendor and length
of the tails arises from the heat which the sun communicates to the comet
as it passes near it. And thence, I think, it may be concluded, that all the
comets that have had such tails have passed very near the sun.

Hence also we may collect that the tails arise from the atmospheres of
the heads (p. 4S7 to 488): but we have had three several opinions about
the tails of comets; for some will have it that they are nothing else but
the beams of the sun’s light transmitted through the comets’ heads, w r hich
they suppose to be transparent; others, that they proceed from the refrac¬
tion which light suffers in passing from the comet’s head to the earth:

556

THE SYSTEM OF THE WORLD.

and, lastly, others, that they are a sort of clouds or vapour constantly
rising from the comets’ heads, and tending towards the parts opposite to
the sun. The first is the opinion of such as are yet unacquainted with
optics; for the beams of the sun are not seen in a darkened room, but in
consequence of the light that is reflected from them by the little particles
of dust and smoke which are always flying about in the air; and hence it
is that in air impregnated with thick smoke they appear with greater
brightness, and are more faintly and more difficultly seen in a finer air;
but in the heavens, where there is no matter to reflect the light, they are
not to be seen at all. Light is not seen as it is in the beams, but as it is
thence reflected to our eyes; for vision is not made but by rays falling
upon the eyes, and therefore there must be some reflecting matter in those
parts where the tails of comets are seen; and so the argument turns upon
the third opinion ; for that reflecting matter can be no where found but in
the place of the tail, because otherwise, since all the celestial spaces are
equally illuminated by the sun’s light, no part of the heavens could appear
with more splendor than another. The second opinion is liable to many
difficulties. The tails of comets are never seen variegated with those
colours which ever use to be inseparable from refraction ; and the distinct
transmission of the light of the fixed stars and planets to us is a demon¬
stration that the aether or celestial medium is not endowed with any re¬
fractive power. For as to what is alledged that the fixed stars have been
sometimes seen by the Egyptians environed with a coma or capillitium
because that has but rarely happened, it is rather to be ascribed to a casual
refraction of clouds, as well as the radiation and scintillation of the fixed
stars to the refractions both of the eyes and air ; for upon applying a tele¬
scope to the eye, those radiations and scintillations immediately disappear.
By the tremulous agitation of the air and ascending vapours, it happens
that the rays of light are alternately turned aside from the narrow space
of the pupil of the eye; but no such thing can have place in the much
wider aperture of the object-glass of a telescope; and hence it is that a
scintillation is occasioned in the former case which ceases in the latter;
and this cessation in the latter case is a demonstration of the regular trans¬
mission of light through the heavens without any sensible refraction.
But, to obviate an objection that may be made from the appearing of no
tail in such comets as shine but with a faint light, as if the secondary
rays were then too weak to affect the eyes, and for this reason it is that
the tails of the fixed stars do not appear, we are to consider that by the
means of telescopes the light of the fixed stars may be augmented above
an hundred fold and yet no tails are seen; that the light of the planets is
yet more copious without any tail, but that comets are seen sometimes
with huge tails when the light of their heads is but faint and dull; for
so it happened in the comet of the year 1680, when in the month of De -

THE SYSTEM OF THE WORLD.

557

cemher it was scarcely equal in light to the stars of the second magnitude,
and yet emitted a notable tail, extending to the length of 40°, 50°, 60°, or
70°, and upwards ; and afterwards, on the 27th and 28th of January , the
head appeared but as a star of the seventh magnitude; but the tail (as
was said above), with a light that was sensible enough, though faint, was
etretched out to 6 or 7 degrees in length, and with a languishing light
that was more difficultly seen, even to 12° and upwards. But on the 9th
and 10th of February , when to the naked eye the head appeared no more,
I saw through a telescope the tail of 2° in length. But farther: if the
tail was owing to the refraction of the celestial matter, and did deviate
from the opposition of the sun, according as the figure of the heavens re¬
quires, that deviation, in the same places of the heavens, should be always
directed towards the same parts: but the comet of the year 1680, Decem¬
ber 28 (l . Sj h . P. M. at Loudon , was seen in Pisces, 8° 41', with latitude
north 2S° 6', while the sun was in Capricorn 18° 26'. And the comet of
the year 1577, December 29, was in Pisces S° 41', with latitude north
2S 3 40' ; and the sun, as before, in about Capricorn 18° 26'. In both
cases the situation of the earth was the same, and the comet appeared in
the same place of the heavens ; yet in the former case the tail of the comet
(as well by my observations as by the observations of others) deviated
from the opposition of the sun towards the north by an angle of 4^ de¬
grees, whereas in the latter there was (according to the observation of
Tycho) a deviation of 21 degrees towards the south. The refraction,
therefore, of the heavens being thus disproved, it remains that the pheno¬
mena of the tails of comets must be derived from some reflecting matter.
That vapours sufficient to fill such immense spaces may arise from the
comet’s atmospheres, may be easily understood from what follows.

It is well known that the air near the surface of our earth possesses a
space about 1200 times greater than water of the same weight; and there¬
fore a cylindric column of air 1200 feet high is of equal weight with a
cylinder of water of the same breadth, and but one foot high. But a
cylinder of air reaching to the top of the atmosphere is of equal weight
with a cylinder of water about 33 feet high; and therefore if from the
whole cylinder of air the lower part of 1200 feet high is taken away, the
remaining upper part will be of equal weight with a cylinder of water 32
feet high. Wherefore at the height of 1200 feet, or two furlongs, the
weight of the incumbent air is less, and consequently the rarity of the
’ compressed air greater, than near the surface of the earth in the ratio of
33 to 32. And, having this ratio, we may compute the rarity of the air
in all places whatsoever (by the help of Cor. Prop. XXII, Book II), sup¬
posing the expansion thereof to be reciprocally proportional to its compres¬
sion ; and this proportion has been proved by the experiments of Hooke
and others. The result of the computation I have set down in the follow-

558

THE SYSTEM CF THE WORLD.

ing table, in the first column of which you have the height o the air in
miles, whereof 4000 make a semi-diameter cf the earth; in the second the
compression of the air, or the incumbent weight; in the third its rarity or
expansion, supposing gravity to decrease in the duplicate ratio of the
distances from the earth’s centre. And the Latin numeral characters
are here used for certain numbers of ciphers, as 0,xvii 1224 for
0,000000000000000001224, and 2695G xv for 26956000000000000000.

AIR’s

Height.

Compression. '

Expansion.

0

33

1

5

17,8515

1.8486

10

9,6717

3.4151

20

2.852

11.571

40

0,2525

136,83

400

O.xvii 1224

26956 xv

4000

O.cv. 4465

73907 oii

40000

O.exeii 1628

20263 clxxxix

400000

O.ecx 7895

41798 ccvii

4000000

0,ccxii 9878

33414 ccix

(Infinite.|

O.eoxii 6041 i

54622 crix

But from this table it appears that the air, in proceeding upwards, is
rarefied in such manner, that a sphere of that air which is nearest to the
earth, of but one inch in diameter, if dilated with that rarefaction which
it would have at the height of one semi-diameter of the earth, would fill all
the planetary regions as far as the sphere of Saturn, and a great way be¬
yond ; and at the height of ten semi-diameters of the earth would fill up
more space than is contained in the whole heavens on this side the fixed
stars, according to the preceding computation of their distance. And
though, by reason of the far greater thickness of the atmospheres of comets,
and the great quantity of the circum-solar centripetal force, it may happen
that the air in the celestial spaces, and in the tails of comets, is not so
vastly rarefied, yet from this computation it is plain that a very small
quantity of air and vapour is abundantly sufficient to produce all the ap¬
pearances of the tails of comets; for that they are indeed of a very notable
rarity appears from the shining of the stars through them. The atmos¬
phere oi the earth, illuminated by the sun’s light, though but of a few miles
in thickness, obscures and extinguishes the light not only of all the stars,
but even of the moon itself; whereas the smallest stars are seen to shine
through the immense thickness of the tails of comets, likewise illuminated
by the sun, without the least diminution of their splendor.

Kepler ascribes the ascent of the tails of comets to the atmospheres of
their heads, and their direction towards the parts opposite to the sun to the
action of the rays of light carrying along with them the matter of the
comets’ tails; and without any great incongruity we may suppose that, in
so free spaces, so fine a matter as that of the aether may yield to the action

THE SYSTEM OF THE WORLD.

559

of the rays of the sun’s light, though those rays are not able sensibly to move
the gross substances in our parts, which are clogged with so palpable a re¬
sistance. Another author thinks that there may be a sort of particles of
matter endowed with a principle of levity as well as others are with a
power of gravity; that the matter of the tails of comets may be of the
former sort, and that its ascent from the sun may be owing to its levity;
but, considering the gravity of terrestrial bodies is as the matter of the
bodies, and therefore can be neither more nor less in the same quantity of
matter, I am inclined to believe that this ascent may rather proceed from
the rarefaction of the matter of the comets’ tails. The ascent of smoke in
a chimney is owing to the impulse of the air with which it is entangled.
The air rarefied by heat ascends, because its specific gravity is diminished,
and in its ascent carries along with it the smoke with which it is engaged.
x4nd why may not the tail of a comet rise from the sun after the same
manner ? for the sun’s rays do not act any way upon the mediums which
they pervade but by reflection and refraction; and those reflecting parti¬
cles heated by this action, heat the matter of the rnther which is involved
with them. That matter is rarefied by the heat which it acquires, and
because by this rarefaction the specific gravity, with which it tended
towards the sun before, is diminished, it will ascend therefrom like a stream,
and carry along with it the reflecting particles of which the tail of the
comet is composed; the impulse of the sun’s light, as we have said, pro¬
moting the ascent. »

But that the tails of comets do arise from their heads (p. 4SS), and tend
towards the parts opposite to the sun, is farther confirmed from the laws
which the tails observe; for, lying in the planes of the comets’ orbits which
pass through the sun, they constantly deviate from the opposition of the
sun towards the parts which the comets’ heads in their progress along those
orbits have left; and to a spectator placed in those planes they appear in
the parts directly opposite to the sun; but as the spectator recedes from
those planes, their deviation begins to appear, and daily becomes greater.
And the deviation, creteris paribus, appears less when the tail is more ob¬
lique to the orbit of the comet, as well as when the head of the comet ap¬
proaches nearer to the sun; especially if the angle of deviation is estimated
near the head of the comet. Farther; the tails which have no deviation
appear straight, but the tails which deviate are likewise bended into a cer¬
tain curvature; and this curvature is greater when the deviation is greater,
and is more sensible when the tail, cccteris paribus, is longer; for in the
shorter tails the curvature is hardly to be perceived. And the angle of
deviation is less near the comet’s head, but greater towards the other end
of the tail, and that because the lower side of the tail regards the parts
from which the deviation is made, and which lie in a right line drawn out
infinitely from the sun through the comet’s head. And the tails that are

560

THE SYSTEM OF THE WORLD.

longer and broader, and shine with a stronger light, appear more resplendent
and more exactly defined on the convex than on the concave side. Upon
which accounts it is plain that the phenomena of the tails of comets de¬
pend upon the motions of their heads, and by no means upon the places of
the heavens in which their heads are seen; and that, therefore, the tails of
the comets do not proceed from the refraction of the heavens, but from
their own heads, which furnish the matter that forms the tail; for as in
our air the smoke of a heated body ascends either perpendicularly, if the
body is at rest, or obliquely if the body is moved obliquely, so in the
heavens, where all the bodies gravitate towards the sun, smoke and vapour
must (as we have already said) ascend from the sun, and either rise perpen¬
dicularly, if the smoking body is at rest, or obliquely, if the body, in the
progress of its motion, is always leaving those places from which the upper
or higher parts of the vapours had risen before. And that obliquity will
be less where the vapour ascends with more velocity, to wit, near the
smoking body, when that is near the sun ; for there the force of the sun by
which the vapour ascends is stronger. But because the obliquity is varied,
the column of vapour will be incurvated; and because the vapour in the
preceding side is something more recent, that is, has ascended something
more lately from the body, it will therefore be something more dense on
that side, and must on that account reflect more light, as well as be better
defined; the vapour on the other side languishing by degrees, and vanish¬
ing out of sight.

But it is none of our present business to explain the causes of the ap*
pearances of nature. Let those things which we have last said be true or
false, we have at least made out, in the preceding discourse, that the rays
of light are directly propagated from the tails of comets in right lines
through the heavens, in which those tails appear to the spectators wherever
placed; and consequently the tails must ascend from the heads of the comets
towards the parts opposite to the sun. And from this principle we may

determine anew the limits of their dis¬
tances in manner following. Let S rep¬
resent the sun, T the earth, STA the
elongation of a comet from the sun, and
ATB the apparent length of its tail;
and because the light is propagated from
the extremity of the tail in the direction
of the right line TB, that extremity
must lie somewhere m the line TB.
Suppose it in D, and join DS cutting
TA in C. Then, because the tail is al -
ways stretched out towards the parts
nearly opposite to the sun, and thereJorc

THE SYSTEM OF THE WORLD.

561

the sun, the head of the comet, and the extremity of the tail, lie in a right
line, the comet’s head will be found in C. Parallel to TB draw SA, meet¬
ing the line TA in A, and the comet’s head 0 must necessarily be found
between T and A, because the extremity of the tail lies somewhere in the
infinite line TB; and all the lines SI) which can possibly be drawn from
the point S to the line TB must cut the line TA somewhere between T
and A. Wherefore the distance of the comet from the earth cannot exceed
the interval TA, nor its distance from the sun the interval SA beyond, or
ST on this side the sun. For instance: the elongation of the comet of
16S0 from the sun, Dec. 12, was 9°, and the length of its tail 35° at least.
If, therefore, a triangle TSA is made, whose angle T is equal to the elon¬
gation 9°, and angle A equal to ATB, or to the length of the tail, viz., 35°,
then SA will be to ST, that is, the limit of the greatest possible distance
of the comet from the sun to the semi-diameter of the orbis rnagmts , as
the sine of the angle T to the sine of the angle A, that is, as about 3 to
11. And therefore the comet at that time was less distant from the sun
than by T 3 T of the earth’s distance from the sun, and consequently either
was within the orb of Mercury, or between that orb and the earth. Again,
Dec. 21, the elongation of the comet from the sun was 32§°, and the length
of its tail 70°. Wherefore as the sine of 321° to the sine of 70°, that is,,
as 4 to 7, so was the limit of the comet’s distance from the sun to the dis¬
tance of the earth from the sun, and consequently the comet had not then
got without the orb of Venus. Dec. 28, the elongation of the comet from
the sun was 55°, and the length of its tail 56° ; and therefore the limit of
the comet’s distance from the sun was not yet equal to the distance of the
earth from the same, and consequently the comet had not then got without
the earth’s orbit. But from its parallax we find that its egress from the
orbit happened about Jan. 5, as well as that it had descended far within
the orbit of Mercury. Let us suppose it to have been in its perihelion
Dec. the 8th, when it was in conjunction with the sun; and it will follow
that in the journey from its perihelion to its exit out of the earth’s orbit
it had spent 2S days; and consequently that in the 26 or 27 days fol¬
lowing, in which it ceased to be farther seen by the naked eye, it had
scarcely doubled its distance from the sun; and by limiting the distances
of other comets by the like arguments, we come' at last to this conclu¬
sion,—that all comets, during the time in which they are visible by us,
are within the compass of a spherical space described about the sun as a
centre, with a radius double, or at most triple, of the distance of the earth
from the sun.

And hence it follows that the comets, during the whole time of their
appearance unto us, being within the sphere of activity of the circum¬
solar force, and therefore agitated by the impulse of that force, will (by
Cor. 1, Prop. XII, Book I, for the same reason as the planets) be made tc

36

562

THE SYSTEM OF THE WORLD.

move in conic sections that have one focus in the centre of the sun. and
by radii drawn to the sun, to describe areas proportional to the times; for
that force is propagated to an immense distance, and will govern the
motions of bodies far beyond the orbit of Saturn.

There are three hypotheses about comets (p. 466); for some will have it
that they are generated and perish as often as they appear and vanish;
others, that they come from the regions of the lixed stars, and are seen by
us in their passage through the system of our planets; and, lastly, others,
that they are bodies perpetually revolving about the sun in very eccentric
orbits. In the first case, the comets, according to their different veL cities,
will move in conic sections of all sorts; in the second, they will describe
hyperbolas, and in either of the two will frequent indifferently all quar¬
ters of the heavens, as well those about the poles as those towards the
ecliptic; in the third, their motions will be performed in ellipses very ec¬
centric, and very nearly approaching to parabolas. But (if the law of the
planets is observed) their orbits will not much decline from the plane of
the ecliptic; and, so far as I could hitherto observe, the third case obtains;
for the comets do, indeed, chiefly frequent the zodiac, and scarcely ever
attain to a heliocentric latitude of 40°. And that they move in orbits
very nearly parabolical, I infer from their velocity ; for the velocity with
which a parabola is described is every where to the velocity with which a
comet or planet may be revolved about the sun in a circle at the same dis¬
tance in the subduplicate ratio of 2 to 1 (by Cor. VII, Prop. XVI); and,
by my computation, the velocity of comets is found to be much about
the same. I examined the thing by inferring nearly the velocities from
the distances, and the distances both from the parallaxes and the pheno¬
mena of the tails, and never found the errors of excess or defect in the ve¬
locities greater than what might have arose from the errors in the dis¬
tances collected after that manner. But I likewise made use of the reason¬
ing that follows.

Supposing the radius of the orbis mogmts to be divided into 1000
parts: let the numbers in the first column of the following table represent
the distance of the vertex of the parabola from the sun’s centre, expressed
by those parts: and a comet in the times expressed in col. 2, will pass
from its perihelion to the surface of the spheie which is described about
the sun as a centre with the radius of the orbis magrnis; and in the
times expressed in col. 3, 4, and 5, it will double, triple, and quadruple,
that its distance from the sun.

THE SYSTEM OF THE WORLD.

563

TABLE L

1 he dis¬
tance of a
comet’s

The time of a oom-et s passage from its p«
distance from the him equal tc

jrihclion to a

from the

The ra it of

Sun’s o^n

the orbis

To its double. 1

To its triple

To its Quad-

tie.

magnus.

ruple.

(1. h. '

d. h. '

d. h. '

d. h. '

0

27 11 12

77 16 28

142 17 14

219 17 30

5

27 16 07

77 23 14

10

27 21 00

78 06 24

20

28 06 40

78 20 13

144 03 19

221 08 54

40

29 01 32

79 23 34

80

30 13 25

82 04 56

160

33 05 29

86 10 26

153 16 08

232 12 20

320

37 13 46

93 23 38

640

37 09 49

105 01 28

1280

106 C6 35

200 06 43

297 03 46

2560

147 22 31

300 06 03

[This table, here corrected, is made on the supposition that the earth’s
diurnal motion is just 59', and the measure of one minute loosely 0,2909,
in respect of the radius 1000. If those measures are taken true, the
true numbers of the table will all come out less. But the difference,
even when greatest, and to the quadruple of the earth’s distance from
the sun, amounts only to 16 h . 55'.]

The time of a comet’s ingress into the sphere of the orb\s magnus , or
of its egress from the same, may be inferred nearly from its parallax, but
with more expedition by the following

TABLE II.

its distance from

The apparent Its apparent diur- the earth in parts
elongation of nal motion in itf> whereof the radius
a comet fr>m own orbit. of th e orbis magnus

the Kun _ contains 1000.

Direct. Ret rug

60° 2° 18' 00° 20’ 1000

65 2 33 00 35 845

70 2 55 00 57 684

) 72 3 07 01 09 618

/4 3 23 01 25 651

76 3 43 01 45 484

78 4 10 02 12 416

80 4 57 02 49 347

82 5 45 03 47 278

84 7 18 05 20 209

i 86 10 27 08 19 140

! 88 18 37 16 39 70

| 90 Infi’ite Infi’ite 00

564

THE SYSTEM OF THE WORLD.

'Fne ingress ot a comet into the sphere of the orbis magnus , or its
egress from the same, happens at the time of its elongation from the sun,
expressed in col. 1, against its diurnal motion. So in the comet of 1681,
Jan. 4, O.S. the apparent diurnal motion in its orbit was about 3° 5', and
the corresponding elongation 71^° ; and the comet had acquired this elon¬
gation from the sun Jan. 4, about six in &e evening. Again, in the year
1680, Nov. 11, the diurnal motion of the comet that then appeared was
about 4|°; and the corresponding elongation 79J happened Nov. 10, a
little before midnight. Now at the times named these comets had arrived
at an equal distance from the sun with the earth, and the earth was then
almost in its perihelion. But the first table is fitted to the earth’s mean
distance from the sun assumed of 1000 parts; and this distance is greater
by such an excess of space as the earth might describe by its annual motion
in one day’s time, or the comet by its motion in 16 hours. To reduce the
comet to this mean distance of 1000 parts, we add those 16 hours to the
former time, and subduct them from the latter ; and thus the former be¬
comes Jan. 4' 1 . 10''. afternoon ; the latter Nov. 10, about six in the morn¬
ing. But from the tenor and progress of the diurnal motions it appears
that both comets were in conjunction with the sun between Dec. 7 and Dec.
8; and from thence to Jan. 4 d . 10 h . afternoon on one side, and to Nov.
10 ‘. 6 h . of the morning on the other, there are about 2S days. And so
many days (by Table 1) the motions in parabolic trajectories do require.

But though we have hitherto considered those comets as two, yet, from
the coincidence of their perihelions and agreement of their velocities, it is
probable that in effect they were but one and the same; and if so, the
orbit of this comet must have either been a parabola, or at least a conic
section very little differing from a parabola, and at its vertex almost in
contact with the surface of the sun. For (by Tab. 2) the distance of the
comet from the earth, Nov. 10, was about 360 parts, and Jan. 4, about
630. From which distances, together with its longitudes and latitudes,
we infer the distance of the places in which the comet was at those times
to have been about 2S0 : the half of which, viz., 140, is an ordinate to the
comet’s orbit, cutting off a portion of its axis nearly equal to the radius
of the orbis magnus , that is, to 1000 parts. And, therefore, dividing the
square of the ordinate 140 by 1000, the segment of the axis, we find the
latus rectum 19, 16, or in a round number 20; the fourth part whereof,
5, is the distance of the vertex of the orbit from the sun’s centre. But the
time corresponding to the distance of 5 parts in Tab. 1 is 27 d . 16 h . 7'. Ir.
which time, if the comet moved in a parabolic orbit, it would have been
carried from its perihelion to the surface of the sphere of the orbis mag -
mis described with the radius 1000, and would have spent the double of
that time, viz., 55 d . 8j h . in the whole course of its motion within that
3phere: and so in fact it did ; for from Nov. 10 d . 6 b . of the morning, the

THE SYSTEM OF THE WORLD.

£>00

lime of the comet’s ingress into the sphere of the orbis magnus , to Jan.
4' 1 . 10 h . afternoon, the time of its egress from the same, there are 55 d . 16 h .
The small difference of 7£ h . in this rude way of computing is to be neg¬
lected, and perhaps may arise from the comet’s motion being some small
matter slower, as it must have been if the true orbit in which it was car¬
ried was an ellipsis. The middle time between its ingress and egress was
December S d . 2 h . of the morning; and therefore at this time the comet
ought to have been in its perihelion. And accordingly that very day, just
before sunrising, Dr. Halley (as we said) saw the tail short and broad, but
very bright, rising perpendicularly from the horizon. From the position
of the tail it is certain that the comet had then crossed over the ecliptic,
and got into north latitude, and therefore had passed by its perihelion,
which lay on the other side of the ecliptic, though it had not yet come into
conjunction with the sun ; and the comet [see more of this famous comet,
p. 475 to 4S6] being at this time between its perihelion and its conjunc¬
tion with the sun, must have been in its perihelion a few hours before;
for in so near a distance from the sun it must have been carried with great
velocity, and have apparently described almost half a degree every hour.

By like computations I find that the comet of 161S entered the sphere
of the orbis magnus December 7, towards sun-setting; but its conjunc¬
tion with the sun was Nov. 9, or 10, about 28 days intervening, as in the
preceding comet; for from the size of the tail of this, in wh ; ch it was
equal to the preceding, it is probable that this comet likewise did come
almost into a contact with the sun. Four comets were seen that year of
which this was the last. The second, which made its first appearance
October 31, in the neighbourhood of the rising sun, and was soon after hid
under the sun’s rays, 1 suspect to have been the same with the fourth,
which emerged out of the sun's rays about Nov. 9. To these we may add
the comet of 1607, which entered the sphere of the orbis magnus Sept.
14, O.S. and arrived at its perihelion distance from the sun about October
19, 35 days intervening. Its perihelion distance subtended an apparent
angle at the earth of about 23 degrees, and was therefore of 390 parts.
And to this number of parts about 34 days correspond in Tab. 1. Far¬
ther ; the comet of 1665 entered the sphere of the orbis magnus about
March 17, and came to its perihelion about April 16, 30 days intervening.
Its perihelion distance subtended an angle at the earth of about seven
degrees, and therefore was of 122 parts : and corresponding to this number
of parts, in Tab. 1, we find 30 days. Again ; the comet of 1682 entered
the sphere of the orbis magnus about Aug. 11, and arrived at its perihe¬
lion about Sep. 16, being then distant from the sun by about 350 parts, to
which, in Tab. I, belong 33^ days. Lastly; that memorable comet of
Regiomontanus , which in 1472 was carried through the circum-polar
parts of our northern hemisphere with such rapidity as to describe 40

566

THE SYSTEM OF THE WORLD.

degrees in one day, entered the sphere of the orbis magnus Jan 21, about
the time that it was passing by the pole, and, hastening from then««
towards the sun, was hid under the sun's rays about the end of Feo. y
whence it is probable that 30 days, or a few more, were spent between its
ingress into the sphere of the orbis magnus and its perihelion. Nor did
this comet truly move with more velocity than other comets, but owed the
greatness of its apparent velocity to its passing by the earth at a near
distance.

It appears, then, that the velocity of comets (p. 471), so far as it can be
determined by these rude ways of computing, is that very velocity with
which parabolas, or ellipses near to parabolas, ought to be described; and
therefore the distance between a comet and the sun being given, the \elocity
of the comet is nearly given. And hence arises this problem.

f PROBLEM.

The relation betwixt the velocity of a comet and its distance from the
sun’s centre being given , the comet’s trajectory is required.

If this problem was resolved, we should thence have a method ot deter
mining the trajectories of comets to the greatest accuracy; for if that re¬
lation be twice assumed, and from thence the trajectory be twice computed,
and the error of each trajectory be found from observations, the assumption
may be corrected by the Rule of False, and a third trajectory may thence
be found that will exactly agree with the observations. And bv deter¬
mining the trajectories of comets after this method, we may come, at last,
to a more exact knowledge of the parts through which those bodies travel,
of the velocities with which they are carried, what sort of trajectories they
describe, and what are the true magnitudes and forms of their tails accord¬
ing to the various distances of their heads from the sun: whether, after
certain intervals of time, the same comets do return again, and in what
periods they complete their several revolutions. But the problem mav be
resolved by determining, first, the hourly motion of a comet to a given time
from three or more observations, and then deriving the trajectory from this
motion. And thus the invention of the trajectory, depending on one ob¬
servation, and its hourly motion at the time of this observation, will either
confirm or disprove itself; for the conclusion that is drawn from the mo¬
tion only of an hour or two and a false hypothesis, will never agree with
the motions of the comets from beginning to end. The method of th<
whole computation is this.

THE SYSTEM OF THE WORLD.

567

LEMMA I.

To cut two right lines OR, TP, given in position , by a third right lint
RP, so as TRP may be a right angle ; and , if another right line SP
is drawn to any given point S, the solid contained under this line SP,
and the square of the right line OR terminated at a given point O,
may be of a given magnitude .

It is done by linear description thus. Let the given magnitude of the
solid be M 2 X N; from any point r of the right line OR erect the per¬

pendicular rp meeting TP in p . Then through the point Sj o draw the
M 2 X N

line S q equal to ——. In like manner draw three or more right lines

S2 q, SS*/, &c.; and a regular line q2q3q, drawn through all the points
q2q3q, (fee., will cut the right line TP in the point P, from which the per¬
pendicular PR is to be let fall. Q.E.F.

By trigonometry thus. Assuming the right line TP as found by the
preceding method, the perpendiculars TR, SB, in the triangles TPR, TPS,
will be thence given; and the side SP in the triangle SBP, as well as the

M 2 X N
error OH“

— SP.

Let this error, suppose D, be to a new error, sup¬

pose E, as the error 2p2q + 3p3q to the error 2p3p ; or as the error 2p2q
+ D to the error 2pP ; and this new error added to or subducted from the
length TP, will give the correct length TP + E. The inspection of the
figure will shew whether we are to add to or subtract; and if at any time
there should be use for a farther correction, the operation may be repeated

668

THE SYSTEM OF THE WORLD.

By arithmetic thus. Let us suppose the thing done, and let TP + e be the
correct length of the right line TP as found out by delineation: and thence

TR

the correct lengths of the lines OR, BP, and SP, will be OR — ^pp e >

BP + e, and v/SP 2 + 2BPe + ee =

M 2 N

20R X TR TR 2
OR 2 + -™- e

TP TP*

BP SB 2

Whence, by the method of converging series, we have SP + gpe -f- -

f M*N 2TR M 2 N 3TR 2 M 2 N , „ , .

ee,&c., = gg 2 + T p X 0 R 3 e + tp^ X OR^ eej<fec ‘ * or the given

M 2 N

co-efficients — SP,

2TR M 2 N BP 3TR 2 M 2 N

SB

OR 2 TP X OR 3 SP ; TP 2 ~ OR 4 2SP 3 ’

X nn.“

F F

F

putting F, —, gg, and carefully observing the signs, we find F + +

F ee

gg ee == 0, and e + g = — G. Whence, neglecting the very small

t- e .

term g, e comes out equal to — G. If the error g is not despicable, take

„ G2

_ G _ g- = e.

And it is to be observed that here a general method is hinted at for
solving the more intricate sort of problems, as well by trigonometry as by
arithmetic, without those perplexed computations and resolutions of affected
equations which hitherto have been in use.

LEMMA II.

To cut three right lines given in position by a fourth right line that
shall pass through a point assigned in any of the three , and so as its
intercepted parts shall be in a given ratio one to the other.

Let AB, AC, BC, be the right lines given in position, and suppose D to
be the given point in the line AC. Parallel to AB draw DG meeting BC

in G; and, taking GF to BG in the given ratio, draw FDE; and FD
will be to 1)E as FG to BG. Q..E.F.

THE SYSTEM OF THE WORLD.

569

By trigonometry thus. In the triangle CGD all the angles and the side
CD are given, and from thence its remaining sides are found; and from
the given ratios the lines GF and BE are also given.

LEMMA III.

To find and represent by a linear description the hourly motion of a comet

to any given time.

From observations of the best credit, let three longitudes of the comet
be given, and, supposing ATR, RTB, to be their differences, let the hourly
motion be required to the time of the middle observation TR. By Lem
IT. draw the right line ARB, so as its intercepted parts AR, RB, may b«

as the times between the observations; and if we suppose a body in the
whole time to describe the whole line AB with an equal motion, and to be
in the mean time viewed from the place T, the apparent motion of that
body about the point R will be nearly the same with that of the comet at
the time of the observation TR.

The same more accurately.

Let Ta, T6, be two longitudes given at a greater distance on one sMe
and on the other; and by Lem. II draw the right line aKb so as its inter¬
cepted parts aR, Kb may be as the times between the observations aTR, RT/>.
Suppose this to cut the lines TA, TB, in D and E; and because the error
of the inclination TRa increases nearly in the duplicate ratio of the time
between the observations, draw FRG, so as either the angle DRF may be
to the angle ARF, or the line DF to the line AF, in the duplicate ratio
of the whole time between the observations aTB to the whole time between
the observations ATB, and use the line thus found FG in place of the
line AB found above.

It will be convenient that the angles ATR, RTB, aTA, BT6, be nc
less than of ten or fifteen degrees, the times corresponding no greater than

57 0

THE SYSTEM OF THE WORLD.

of eight or twelve days, and the longitude^ taken when the comet moves
with the greatest velocity; for thus the errors of the observation will
bear a less proportion to the differences of the longitudes.

LEMMA IV.

To find the longitudes of a comet to any given times.

It is done by taking in the line FG the distances Rr, Rp, proportional
to the times, and drawing the lines Tr, Tp. The way of working by
trigonometry is manifest.

LEMMA V.

To find the latitudes.

On TF, TR, TG, as radiuses, at right angles erect ¥f RP, Go-, tan¬
gents of the observed latitudes ; and parallel to fg draw PH. The per
pendiculars rp, pti, meeting PH, will be the tangents of the sought latitudes
to Tr and Tp as radiuses.

PROBLEM I.

From the assumed ratio of the velocity to determine the trajectory oj a

comet.

Let S represent the sun ; t, T, r, three places of the earth in its orbit
at e^ual distances; p, P, ti, as many corresponding places of the comet in

its trajectory, so as the distances interposed betwixt place and place may
answer to the motion of one hour; pr, PR, cop, perpendiculars let fall on
the plane of the ecliptic, and rRp the vestige of the trajectory in this
plane. Join S p, SP, Sdi, SR, ST, tr, TR, t p, TP , and let tr, ~p , meet in
O, TR will nearly converge to the same point O, or the error will be in
considerable. By the premised lemmas the angles rOR, ROp, are given,
as well as the ratios pr to tr, PR to TR, and up to rp. ^he figure /TrO

THE SYSTEM OF THE WORLD.

571

is likewise given both in magnitude and position, together with the dis¬
tance ST, and the angles STR, PTR, STP. Let us assume the velocity
of the comet in the place P to be to the velocity of a planet revolved
about the sun in a circle, at the same distance SP, as V to 1 ; and we shall
have a line y?Pw to be determined, of this condition, that the space pd)<
described by the comet in two hours, may be to the space Y X tr (that is.
to the space which the earth describes in the same time multiplied by the
number Y) in the subduplicate ratio of ST, the distance of the earth from
the sun, to SP, the distance of the comet from the sun; and that the space
pP, described by the comet in the first hour, may be to the space Pd>, de¬
scribed by the comet in the second hour, as the velocity in p to the velocity
in P ; that is, in the subduplicate ratio of the distance SP to the distance
S/7, or in the ratio of 2S p to SP + S p ; for in this whole work I neglect
small fractions that can produce no sensible error.

In the first place, then, as mathematicians, in the resolution of affected
equations, are wont, for the first essay, to assume the root by conjecture,
so, in this analytical operation, I judge of the sought distance TR as I
best can by conjecture. Then, by Levi. II. I draw rp, first supposing rR
equal to Rp, and again (after the ratio of SP to Sp is discovered) so as
rR may be to Rp as 2SP to SP + Sp, and I find the ratios of the lines
pd>, rp, and OR, one to the other. Let M be to V X tr as OR to pd >; and
because the square of pd> is to the square of Y X ^ as ST to SP, we
shall have, ex cequo , OR 2 to M 2 as ST to SP, and therefore the solid
OR 2 X SP equal to the given solid M 2 X ST; whence (supposing the
triangles STP, PTR, to be now placed in the same plane) TR, TP, SP,
PR, will be given, by Lem. I. All this I do, first by delineation in a rude
and hasty w r ay ; then by a new delineation with greater care; and, lastly,
by an arithmetical computation. Then I proceed to determine the position
of the lines rp, pd>, with the greatest accuracy, together with the nodes and
inclination of the plane Spw to the plane of the ecliptic; and in that
plane Spti I describe the trajectory in which a body let go from the place
P in the direction of the given right line pti would be carried with a velo¬
city that is to the velocity of the earth as jdw to Y X tr. Q.E.F.

PROBLEM II.

To correct the assumed ratio of the velocity and the trajectory thence

found.

Take an observation of the comet about the end of its appearance, or
any other observation at a very great distance from the observations used
before, and find the intersection of a right line drawn to the comet, in that
observation with the plane Spw, as well as the comet’s place in its trajec¬
tory to the time of the observation. If that intersection happens in this
place, it is a proof that the trajectory was rightly determined; if other-

572

THE SYSTEM OF THE WORLD.

wise, a new number Y is to be assumed, and a new trajectory to be found;
&ul then tin place of the comet in this trajectory to the time of that pro¬
batory observation, and the intersection of a right line drawn to the comet
with the plane of the trajectory, are to be determined as before; and by
comparing the variation of the error with the variation of the other quan¬
tities, we may conclude, by the Rule of Three, how far those other
quantities ought to be varied or corrected, so as the error may become as
small as possible. And by means of these corrections we may have the
trajectory exactly, providing the observations upon which the computation
was founded were exact, and that we did not err much in the assumption
of the quantity V; for if we did, the operation is to be repeated till the
trajectory is exactly enough determined. Q.E.F.

*NJ> OF THE SYSTEM OF THE WORLD.

CONTENTS

OF

THE SYSTEM OF THE WORLD.

That the matter of the heavens is fluid,. 511

The principle of circular motion In free spaces,. . 5 pj

The effects of centripetal forces,.. 512

The certainty of the argument,..

Wh-t follows from the supposed diurnal motion of the stars,. 514

The incongruous consequences of this supposition..

That there is a centripetal force really directed to the centre of every planet, . . . 515

O Centripetal forces decrease in duplicate proportion of distances from the centre of every planet, 5l(j
That the superior p.anets are revolved about the sun, and by radii drawn to the sun describe

areas proportional to the times,. 517

That the force which governs the superior planets is directed not to the earth, but to the sun, . 51i>
That the oiicum-solar force throughout all the regions of the planets decreaseth in the duplicate

proportion of the distances from the sun, ...

That the circum-terrestrial force decreases in the duplicate proportion of the distances from the

earth proved in the hypothesis of the earth’s being at rest,.519

The same proved in the hypothesis of the earth’s motion,.520

The decrement of the forces in the duplicate proportion of the distances from the earth and plan¬
ets, proved from the eccentricity of the planets, and the very slow motion of their apses, . 520
The quantity of the forces tending towards the several planets: the circum-solar very great, . 521

The circum-terrestrial force very small, ... ... ..... 521

The apparent diameters of the planets,. 521

The correction of the apparent diameters,.522

Why the density is greater in some of the planets and less in others; but the forces in all are as

their quantities of matter,.524

Another analogy between the forces and bodies, proved in the celestial bodies, .... 525

Proved in terrestrial bodies,.525

The affinity of those analogies,.526

And coincidence, . * ... 526

That the forces of small bodies are insensible,.527

Which, notwithstanding, there are forces tending towards all terrestrial bodies proportional to

their quantities of matter,.528

Proved that the same forces tend towards the celestial bodies,.528

That from the surfaces of the planets, reckoning outward, their forces decrease in tha duplicate;

but, reckoning inward, in the simple proportion of the distances from their centres, . 52;.

The quantities of the forces and of the motions arising in the several cases, .... 52b

That all the planets revolve about the sun.523

That the common centre of gravity of all the planets is quiescent. That the sun is agitated

with a very slow motion. This motion defined, ..531

That the planets, nevertheless, are revolved in ellipses having their foci in the sun; and by radii

drawn to the sun describe areas proportional to the times,.541

Jf the dimensions of the orbits, and of the motions of their aphelions and nodes, . . . 532

All the motions of the moon that have hitherto been observed by astronomers derived from the

foregoing principles,.532

As also some other unequable motions that hitherto have not been observed, .... 533

And the distance of the moon from the earth to any given time,.533

The motions of the satellites of Jupiter and Saturn derived from the motions of our moon, . 534
That the planets, in respect of the fixed stars, are revolved by equable motions about their

proper axes. And that (perhaps) those motions are the most fit for the equation of time, 534
The moon likewise is revolved by a diurnal motion about its axis, and its libration thence arises, 535
That the sea ought twice to flow, and twice to ebb, every day; that the highest water must fall

out in the chird hour after the appulse of the luminaries to the meridian of the place, . 535

574

CONTENTS OF THE SYSTEM OF THE WORLD.

Tke precession of the equinoxes, and the libratory motion of the axes of the earth and planet , 535
That the greatest tides happen in the syzygies of the luminaries, the least in their quadratures;
and that at the third hour after the appulsc of the moon to the meridian of the place. Bat
that out of the syzygies and quadratures those greatest and least tides deviate a little from
that third hour towards the third hour after the appulse of the sun to the meridian, . 536

That the tides are greatest when the luminaries are in their perigees,.536

That the tides are greatest about the equinoxes,.536

That out of the equator the tides are greater and less alternately, . .... 537

That, by the conservation of the impressed motion, the difference of the tides is diminished ; and

that hence it may happen that the greatest menstiual tide will be the third after the syzygy, 538
That the motio is of the sea may be retarded by impediments in its channels, .... 53S
That from the impediments of channels and shores various phenomena do arise, as that the sea

may flow but once every day,.539

That the times of the tides within the channels of rivers are more unequal than in the ocean, . 540
'1 hat the tides are greater in greater and deeper seas; greater on the shores of continents than
i t islands in the middle of the sea; and yet greater in shallow bays that open with wide

inlets to the sea,. *.540

The force of the sun to disturb the motions of the moon, computed from the foregoing principks, 542

The force of the sun to move the sea computed,.543

The height of the tide under the equator arising from the force of the sun computed, . . 543

The height of the tides under the parallels arising from the sun’s force computed, . . . 544

The proportion of the tides uuder the equator, in the syzygies and quadratures, arising from the

joint forces of both sun and moon,.545

The force of the moon to excite tides, and the height of the water thence arising, computed, . 545

That those forces of the sun and inoon are scarcely sensible by any other effect beside the tidts

which they raise in the sea,.546

That the body of the moon is about six times more dense than the body of the sun, . . . 547

That the moon is more dense than the earth in a ratio of about three to two, .... 547

Of the distance ot the fixed stars,.547

That the comets, as o ten as they become visible to us, are nearer than Jupiter, proved from

their parallax in longitude,.548

The same proved from their parallax in latitude,.549

The same proved otherwise by the parallax,.550

From the light of the comets’ heads it is proved that they descend to the orbit of Saturn, . 550

And also below the orb of Jupiter, and sometimes below the orb of the earth, . . . 551

The same proved from the extraordinary splendor of their tails when they are near the sun, . 551

The same proved from the light of their heads, as being greater, cceteris paribus, when they

come near to the sun,.553

The same confirmed by the great number of comets seen in the region of the sun, . . . 555

This also confirmed by the greater magnitude and splendor of the tails after the conjunction of

the heads with the sun than before,.555

That the tails arise from the atmospheres of the comets,.556

That the air and vapour in the celestial spaces is of an immense rarity ; and that a small quan¬
tity of vapour may be sufficient to explain all the phenomena of the tails of comets, . . 558

After what manner the tails of comets may arise from the atmospheres of their heads. . . 559

That the tails do indeed arise from those atmospheres, proved from several of their phenomena, 559
That comets do sometimes descend below the orbit of Mercury, proved from their tails, . 560

That the comets move in conic sections, having one focus in the centre of the sun, and by radii

drawn to that centre do describe areas proportional to the times,.

That those conic sections are near to parabolas, proved from the velocity of the comets,

In what space of time comets describing parabolic trajectories pass through the sphere of the

orbis magnus, . .

At what time comets enter into and pass out of the sphere of the orbis magnus, .

With what velocity the comets of 1680 passed through the sphere of the orbis magnus,

That these were not two, but one and the same comet. In what orbit and with what velocity
this comet was carried through the heavens described more exactly, ....
With what velocity comets are carried, shewed by more examples, . ...

The investigation of the trajectory of comets proposed, .... ...

Lemmas premised to the solution of the problem,. ...

The problem resolved, . . . • ...

561

361

562

563

INDEX TO THE PRINC1PIA.

3uio/noxes, their praecession—the cause of that motion shewn, . . .... 413

“ the quantity of that motion computed from the causes,.45J

Air, its density at any height, collected by Prop. XXII, Book II, and its density at the height

of one semi-diameter of the earth, shewn,.‘189

its elastic force, what cause it may be attributed to,.302

its gravity compared with that of water,.48‘t

“ its resistance, collected by experiments of pendulums,.313

“ the same more accurately by experiments of falling bodies, and a theory, .... 353
Angles of contact not all of the same kind, but some infinitely less than others, . . . 101

Apsides, their motion shewn,.172, 173

Areas which revolving bodies, by radii drawn to the centre of force describe, compared with the

times of description,. 103, 105, 106, 195, 200

As t the mathematical signification of this word defined,.. . .100

Attraction of all bodies demonstrated,.397

“ the certainty of this demonstration shewn, ..384

“ the cause or manner thereof no where defined by the author, .... 507

“ the common centre of gravity of the earth, sun, and all the planets, is at rest, con¬
firmed by Cor. 2, Prop. XIV, Book III,.401

“ the common centre of gravity of the earth and moon goes round the orbis magnus, 402

“ its distance from the earth and from the moon,.452

Centre, the common centre of gravity of many bodies does not alter its state of motion or rest

by the actions of the bodies among themselves,.87

“ of the forces by which revolving bodies are retained in their orbits, how indicated by

the description of areas,. .107

“ how found by the given velocities of the revolving bodies,.110

Circle, by what law of centripetal force tending to any given point its circumference may be

described, . .108,111,114

Comets, a sort of planets, not meteors,. 465, 486

“ higher than the moon, and in the planetary regions,. 460

“ their distance how collected very nearly by observations,.461

“ more of them observed in the hemisphere towards the sun than in the opposite hemis¬
phere ; and how this comes to pass,.464

“ shine by the sun’s light reflected from them,.464

“ surrounded with vast atmospheres,. 463, 465

“ those which come nearest to the sun probably the least, ... . 495

“ why they are not comprehended within a zodia , like the planets, but move differently

into all parts of the heavens, ... .502

may sometimes fall into the sun, and afford a new supply of fire,.502

“ the use of them hinted,.492

“ move in conic sections, having their foci in the sun’s centre, and by radii drawn to the
sun describe areas proportional to the times. Move in ellipses if they ccme round again

in their orbits, but these ellipses will be near to parabolas,. 466

Comet’s parabolic trajectory found from three observations given,.472

corrected when found, ... .495

“ place in a parabola found to a given time,.466

u velocity compared with the velocity of the planets, .... . . 466

Comets’ Tails directed from the sun,.. 489

“ brightest and largest immediately after their passage through the neighbour¬
hood of the sun,.*.487

u “ their wonderful rarity,.490

“ their origin and nature. .... 463

c “ in what space of time they ascend from their heads, . • 4 99

'j '

n ' _, '

576

INDEX TO THE PRINCIPIA

rfoMET of the years 1664 and 1665—tbe observations of its motion compared with the theory, . 496

“ of tbe years 1680 and 1681—observations of its motion,.474

“ its motion computed in a parabolic orbit,.478

“ in an elliptic orbit,.479

“ its trajectory, and its tail in the several parts of its orbit, delineated, .... 484

" of the year 1682—its motion compared with the theory,. 500—

seems to have appeared in the year 1607, and likely to return again after a period of

75 years,. 501, 502

u of the year 1683—its motion compared with the theory,.499

“ of the year 1723—its motion compared with the theory,.501

Conic Sections, by what law of centripetal force tending to any given point they may be de¬
scribed by revolving bodies,.125

“ the geometrical description of them when the foci are given, .... 125

“ when the foci are not given,.131

“ when the centres or asymptotes are given,.147

Curvature of figures how estimated,. 271, 423

Curves distinguished into geometrically rational and geometrically irrational, . . . 157

Cycloid, or Epicycloid, its rectification,.184

“ “ its evoluta,.185

Cylinder, the attraction of a cylinder composed of attracting particles, whose forces are recip¬
rocally as the square of the distances,.239

Descent of heavy bodies in vacuo, how much it is,.405

“ and ascent of bodies in resisting mediums,. 252, 265, 281, 283, 345

Descent or Ascent rectilinear, the spaces described, the times of decription, and the velocities
acquired in such ascent or descent, compared, on the supposition of any

kind of centripetal force,.160

Earth, its dimension by Norwood , by Picart, and by Cassini, .405

“ its figure discovered, with the proportion of its diameters, and the measure of the degrees

upon the meridian,.• . 405,40?)

“ the excess of its height at the equator above its height at the poles, . . . 407, 412

“ its greatest and least semi-diameter,.407

“ its mean semi-diameter,.407

“ the globe of the earth more dense than if it was entirely water,.400

“ the nutation of its axis,.413

“ the annual motion thereof in tbe orbis magnus demonstrated,.498

“ the eccentricity thereof how much,.452

“ the motion of its aphelion how much,.404

Ellipses, by what law of centripetal force tending to the centre of the figure it is described by a

revolving body,.114

“ by what law of centripetal force tending to the focus of the figure it is described by a

revolving body,.116

Fluid, the definition thereof,.108

Fluids, the laws of their density and compression shewn,.293

“ their motion in running out at a hole in a vessel determined, . . . . . 331

Forces, their composition and resolution,.84

“ attractive forces of spherical bodies, composed of particles attracting according to any

law, determined,.218

“ attractive forces of bodies not spherical, composed of particles attracting according to

any law, determined,.233

“ the invention of the centripetal forces, when a body is revolved in a non-resisting space

about an immoveable centre in any orbit,.103,116

u the centripetal forces tending to any point by which any figure may be described by a
revolving body being given, the centripetal forces tending to any other point by which
the same figure may be described in the same periodic time are also given, . . . US

v the centripetal forces by which any figure is described by a revolving body being given,
there are given the forces by which a new figure may be described, if the ordinates are
augmented or diminished in any given ratio, or the angle of their inclination be any

how changed, the periodic time remaining the same,.US

u centripetal forces decreasing in the duplicate proportion of the distances, what figures

may be described by them,.120 196

INDEX TO THE PRINCIPIA.

577

Force, centripetal force defined,.74

“ the absolute quantity of centripetal force defined,.75

K the accelerative quantity of the same defined,.76

n the motive quantity of the same defined,.76

M the proportion thereof to any known force how collected,.103

“ a centripetal force that is reciprocally as the cube of the ordinate tending to a vastly

remote centre of lorce will cause a body to move in any given conic section, . . 114

“ a centripetal force that is as the cube of the ordinate tending to a vastly remote centre of

force will cause a body to move in an hyperbola,.243

“ centrifugal force of bodies on the earth’s equator, how great,.405

Bod, his nature,.506

Bravity mutual between the earth and its parts,.94

'* of a different nature from magnetical force,.397

“ the cause of it not assigned,.507

“ tends towards all the planets,.393

“ from the surfaces of the planets upwards decreases in the duplicate ratio of the dis¬
tances from the centre,.400

“ from the same downwards decreases nearly in the simple ratio of the same, . . 400

“ tends towards all b idies, and is proportional to the quantity of matter in each, . 397

“ is the force by which the moon is retained in its orbit,.391

“ the same proved by an accurate calculus,.153

K is the force by which the primary planets and the satellites of Jupiter and Saturn are

retained in their orbits,.393

Heat, an iron rod increases in length by heat,.112

“ of the sun, how great at different distances from the sun,.486

“ how great in Mercury,.400

“ how great in the comet of 1680, when in its perihelion, ... ... 486

Heavens are void of any sensible resistance, 401,445, 492; and, therefore, of almost any cor¬
poreal fluid whatever,. 355 356

“ suffer light to pass through them without any refraction, .... 485

Hydrostatics, the principles thereof delivered, . .... 293

Hyperbola, by what law of centrifugal force tending from the centre of the figure it is described

by a revolving body,.116

“ by what law of centrifugal force tending from the focus of the figure it is described

by a revolving body,.117

“ by what law of oe itripetal force tending to the focus of the figure it is described

by a revolving body,.118

Hypotheses of what kind • oever rejected from this philosophy,.508

Iupiter, its periodic time,.388

“ its distance from the sun,.388

“ its apparent diameter,.386

“ its true diameter,. . 399

“ its attractive fi rce, how great, . ... . 398

“ the weights of b< dies on its surface,. . 399

“ its density, ... . . 399

“ its quantity of matter,. . 399

“ its perturbation by Saturn, how much,. 403

“ the proportion of its diameters exhibited by computation, • • • . • 409

“ and compared with observations, . . .. . 409

“ its rotation about its axis, in what time performed, 409

“ the cause of its belts hinted at,.. . . 445

fjlOHT, its propagation not instantaneous,.246

“ its velocity different in different mediums, ... 2-J5

“ a certain reflection it sometimes suffers explained,.245

“ its refraction explained,.243

M refraction is not made in the single point of incidence,.247

“ an incurvation of light about the extremities of bodies observed by experiments, . . 246

“ not caused by the agitation of any ethereal medium, 368

Magnetic force,. . 94,304, 397, 454

> * ^ 37

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INDEX TO THE PRINCIPIA.

Wars, its periodic time,. _

“ its distance from the sun, .. t ggg

“ the motion of its aphelion,. # ^

Matter, its quantity of matter defined,. .... 73

“ its vis insita define i.. ... 74

“ its impressed force defined,. 74

“ its extension, hardness, impenetrability, mobility, vis inertias , gravity, how discovered, 385

“ subtle matttr of Descartes ii quired into,.320

Mechanical Powers explained and demonstrated,..

Mercury, its periodic time,..

“ its distance from the sun,..

“ the motion of its aphelion,..

Method of first and last ratios,..

“ transforming figures into others of the same analytical order,

“ of fluxions,.

*' differential,.

“ of finding the quadratures of all curves very nearly true, .

“ of converging series applied to the solution of difficult problems,

. p5
141
. 261
447
. 448
271,436

Moon, the inclination of its orbit to the ecliptic greatest in the syzygies of the node with the sun,

and least in the quadratures,. 208

“ the figure of its body collected by calculation,. 454

its librations explained,..

“ its mean apparent diameter,. ... 453

“ its true diameter,..

“ weight of bodies on its surface,. 453

“ its density,..

“ its quantity of matter,. 453

“ its mean distance from the earth, how many greatest semi-diameters of the earth con¬
tained therein,..

“ how many mean semi-diameters,. 454

“ its force to move the sea how great,. 449

“ not perceptible in experiments of pendulums, or any statical or hydrostatical observations, 452

“ its periodic time,. 454

“ the time of its synodical revolution,.422

“ its motions, and the inequalities of the same derived from their causes, . . 413, 444

“ revolves more slowly, in a dilated orbit, when the earth is in its perihelion; and more

swiftly in the aphelion the same, its orbit being contracted, .... 413, 444 , 445
“ revolves more slowly, in a dilated orbit, when the apogseon is in the syzygies with the sun ;

and more swiftly, in a contracted orbit, when the apogaeon is in the quadratures, . 445

“ revolves more slowly, in a dilated orbit, when the node is in the syzygies with the sun ;

and more swiftly, in a contracted orbit, when the node is in the quadratures, . . 446

“ moves slower in its quadratures with the sun, swifter in the syzygies; and by a radius
drawn to the earth describes an area, in the first case less in proportion to the time, in the

last case greater,. ... 413

“ the inequality of those areas computed, .... 420

“ its orbit is more curve, and goes farther from the earth in the first case; in the last case

its orbit is less curve, and comes nearer to the earth,.415

“ the figure of this orbit, and the proportion of its diameters collected by computation, . 423
“ a method of finding the moon’s distance from the earth by its horary motion, . . 423

“ its apogseon moves more slowly when the earth is in its aphelion, m< re swiftly in the peri¬
helion, .414,445

“ its apogseon goes forward most swiftly when in the syzygies with the sun; and goes back¬
ward in the quadratures,.414,44C

“ its eccentricity greatest when the apogseon is in the syzygies with the sun ; least when the

same is in the quadratures,.414, 44C

v its nodes move more slowly when the earth is in its aphelion, and more swiftly in the peri¬
helion, .414,445

“ its nodes are at rest in their syzygies with the sun, and go back most swiftly in the quad¬
ratures ... 414

INDEX TO THE PRINCIPIA.

579

Moon, the motions of the nodes and the inequalities of its motions computed from the theory of

gravity,. 427,430, 434, 436

“ the same from a different principle,.437

« the variations of the inclination computed from the theory of gravity, . . . 441, 443

« the equations of the moon’s motions for astronomical uses,.445

“ the annual equation of the moon’s mean motion,.445

“ the first semi-annual equation of the same, . . . . • ... 446

“ the second serni-annual equation of the same,.447

“ the first equation of the moon’s centre,.447

“ the second equation of the moon’s centre,. • . 448

Moon’s first variation,.425

“ the annual equation of the mean motion of its apogee,.445

“ the semi-annual equation of the same,.447

“ the semi-annual equation of its eccentricity, .447

“ the annual equation of the mean motion of its nodes,.445

“ the semi-annual equation of the same,.437

“ the semi-annual equation of the inclination of the orbit to the ecliptic, • . . 444

“ the method of fixing the theory of the lunar motions from observations, . . . 464

Motion, its quantity defined,.73

“ absolute and relative, . . . ..78

“ absolute and relative, the separation of -one from the other possible, demonstrated by

an example ..82

“ laws thereof.83

of concurring bodies after their .reflection, by what experiments collected, ... 91

“ of bodies in eccentric sections, . . .116

“ in moveable orbits, ..172

“ in given superficies, and of the reciprocal motion of pendulums, .... 183

“ ©f bodies tending to each other with oentripetal forces,.194

“ of very small bodies agitated by centripetal forces tending to each part of some very

great body,.233

“ of bodies resisted in the ratio of the velocities,.251

“ in the duplicate ratio of the velocity,.258

“ partly in the simple and partly in the duplicate ratio of the same, .... 280
“ of bodies proceeding by their vis vnsita alone in resisting mediums, 251,258,259, 280, 281, 330
“ of bodies ascending or descending in right lines in resisting mediums, and acted on by

an uniform force of gravity,. 252, 265,2S1, 283

“ of bodies projected in resisting mediums, and acted on by an uniform force of gravity, 255, 268

u of bodies revolving in resisting mediums,.287

“ of funependulous bodies in resisting mediums, . 301

“ and resistance of fluids,.. 323

“ propagated through fluids, ...... 356

“ of fluids after the manner of a vortex, or circular,.370

Motions, composition and resolution of them,.84

Ovals for optic uses, the method of finding them which Cartesius concealed, .... 246

“ a general solution of Cartesius’s problem,. 247, 248

Obbits, the invention of those which are described by bodies going off from a given place with
a given velocity according to a given right line, when the centripetal force is recipro¬
cally as the square of the distance, and the absolute quantity of that force is known, . 123
u of those whieh are described by bodies when the centripetal force is reeiproeally as the

cube of the distance,.114,171,176

“ of those which are described by bodies agitated by any centripetal forces whatever, 168
Parabola., by what law of oentripetal force tending to the focus of the figure the same may be

described,.120

Pendulums, their properties explained,. 186,190, 304

“ the diverse lengths of isochronous pendulums in different latitudes compared among

themselves, both by observations and by the theory of gravity, . . 409 to 413

Place defined, and distinguished into absolute and relative,. .78

Places of bodies moving in conic sections found to any assigned time,.153

*ianets not carried about by corporeal vortices,.378

i> SO

INDEX TO THE PRINCIPIA.

Planet*, their periodic ^iines,. . . $£

“ ilieir distances from the sun,. . . 389

“ the a^heiia and nodes of their orbits do almost rest,.405

“ their orbits determined,.406

“ the way of finding their places in their orbits,. 347 to 350

“ their density suited to the heat they receive from the sun, ...... 400

“ their diurnal revolutions equable,.406

“ their axes less than the diameters that stand upon them at right angles, . . . 406

Planets, Primarv, surround the sun,.387

“ move in ellipses whose foeus is in the sun’s centre,.403

by radii drawn to the sun describe areas proportional to the times, . 388, 403
“ revolve jn periodic times that are in the sesquiplicate proportion of the dis¬
tances from the sun,.387

“ are retained in their orbit3 by a force of gravity which respects the sun,

and is reeiprocally as the square of the distance from the sun’s centre, 389, 393
Planets, Secondary, move in ellipses having their focus in the centre of the primary, . 413

“ by radii drawn to their primary describe areas proportional to the

times. 386,387,390

“ revolve in periodic times that are in the sesquiplicate proportion of their

distances from the primary, . . 386,387

Problem Keplerian, solved by the trochoid and by approximations, .... 157 to 160

of the ancients, of four lines, related by Pappus, and attempted by Car-
tesius, by an algebraic calculus solved by a geometrical composition, . 135

Projectiles move in parabolas when the resistance of the medium is taken away, 91,115, 243, 273

“ their motions in resisting mediums,. 255,268

Pulses of the air, by which sounds are propagated, their intervals or breadths determined, 368, 370
“ these intervals in sounds made by open pipes probably equal to twice the length of the

Pipes,.370

Quadratures general of oval figures not to be obtained by finite terms,.153

Qualities of bodies how discovered, and when to be supposed universal, .... 38-1

Resistance, the quantity thereof in mediums not continued,.329

“ in continued mediums,.40f

“ in mediums of any kind whatever, •.33.

of mediums is as their density, cceteris paribus, . . 320, 321, 324, 329, 344, 355

is in the duplicate proportion of the velocity of the bodies resisted, cceteris pari¬
bus, . 258, 314,324, 329, 344, 351

u is in the duplicate proportion of the diameters of spherical bodies resisted, cceteris

paribus . 317, 318, 329, 34-1

“ of fluids threefold, arises either from the inactivity of the fluid matter, or the te¬
nacity of its parts, or frietion,.286

“ the resistance found in fluids, almost all of the first kind, .... 321,354

“ cannot be diminished by the subtilty of the parts of the fluid, if the density remain, 355

“ of a globe, what proportion it bears to that of a cylinder, in mediums not continued, 327

“ in compressed mediums,.343

“ of a globe in mediums not continued,.329

“ in compressed mediums,.344

“ how found by experiments,. 345 to 355

“ to a frustum oi a cone, how made the least possible,.328

“ what kind of solid it is that meets with the least,.329

Resistances, the theory thereof confirmed by experiments of pendulums, . . . 313 to 321

“ by experiments of falling bodies,. 345 to 356

Rest, true and relative,.78

Rules of philosophy,.384

Satellites, the greatest heliocentric elongation of Jupiter’s satellites,.387

“ the greatest heliocentric elongation of the Huy genian satellite from Saturn’s centre, 398

“ the periodic times of Jupiter’s satellites, and their distances from his centre, . 386, 387

“ the periodic times of Saturn’s satellites, and their distances from his centre, 387, 388

“ the inequalities of the motions of the satellites of Jupiter and Saturn derived from

the motions of the moon,.413

Sm^uiplicate proportion defined,. 101

INDEX TO THE PRINCIPLE

Saturn, its periodic tune,. . 388

“ its distance from the sun,. .388

“ its apparent diameter.388

K its true diameter,.399

“ its attractive force, how great,.398

“ the weight of bodies on its surface,.399

“ its density, ... . . . 399

“ it3 quantity of matter,. 399

“ its perturbation by the approach of Jupiter how great,.403

“ the apparent diameter of its ring,.388

Shadow of the earth to be augmented in lunar eclipses, because of the refraction of the at¬
mosphere, .447

SuUnds, their nature explained, . 360,363, 365, 366, 367,368, 369

not propagated in directum, . . ... 359

“ caused by the agitation of the air,.368

“ their velocity c< mputed,. 368, 369

“ somewhat swifter by the theory in summer than rn winter,.370

“ cease immediately, when the motion of the sonorous body ceases, .... 365

“ how augmented in speaking trumpets,.370

Space, absolute and relative,.78, 79

“ not equally full..396

Spheroid, the attraction of the same when the forces of its particles are reciprocally as the

squares of the distances,.239

Spiral cutting all its radii in a given angle, by what law of centripetal force tending to the

centre thereof it may be described by a revolving body, .... 107,287,291

Spirit pervading all bodies, and concealed within them, hinted at, as required to solve a great

many phenomena of Nature,.508

Stars, the fixed star3 demonstrated to be at rest, ... .404

“ their twinkling what to be ascribed to,. .487

“ new stars, whence they may arise,.502

Substances of all things unknown,.507

Sun, moves round the common centre of gravity of all the planeta,.401

“ the periodic time or' its revolution about its axis,.405

“ its mean apparent diameter,.•.. 453

“ its true diameter,. 398

“ its horizontal parallax,.398

“ has a menstrual parallax.403

“ its attractive force bow great,.398

“ the weight <.f bodies on its surface, .399

“ its density,...* 399

“ its quantity of matter,.399

“ its force to disturb the motions of the moon,.391, 419

“ its force to move the sea,.448

Tides of the sea derived from their cause,. 415, 448, 449

Time, absolute and relative,.78, 79

“ the astronomical equation thereof proved by pendulum clocks, and the eclipses of Jupiter’s

satellites,.79

A Vacuum proved, or that all spaces (if said to be full) are not equally full, .... 396
Velocities of bodies moving in conic sections, where the centripetal force tends to the focus, . 121
Velocity, the greatest that a globe falling in a resisting medium can acquire, . . . 314

Venus, its periodic time,.388

“ its distance from the sun,.388

“ the motion of its aphelion, ..405

Vortices, their nature and constitution examined,.504

Waves, the velocity with which they are propagated on the superficies of stagnant water, . 361
Weights of bodies towards the sun, the earth, or any planet, are, at equal distances from the 1

centre, as the quantities of matter in the bodies,.394

'* they do not depend upon the forms and textures of bodies.395

4 ‘ of bodies in different regions of the earth found out, and compared together, . . 409

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